I have to say, this method worked for me instantly, however, after 4 months of experiences, good and bad ones and a lot more I've learned along the way, compared to when I was a total newbie to these things back then, I would have done some things differently and have been more critical to using the letter method.
This post is not to make anyone or anything look bad, it's just my personal opinion about the specific method and only, and a guide/warning to those new who think to use it, but not being completely aware of the meaning of the ritual and possible consequences.
Let me talk a little first about what I experienced, before I explain things, for you to understand better.
The days before I wanted to do the letter method (kinda from the moment I decided I would do it), I felt more and more sexually loaded, as if something would be charging in me. I didn't do anything for relieve though (you know what I mean, do I need to explain? hehe). Then a day before deciding to do it the next night, I made a draft of what I would write in that letter. The next day however, I felt all this sexual tension completely gone, I decided I would perform the method in the night anyway, even though I thought it would have been helpful to had that sexual "chargeup". Honestly, I have no idea if the draft I wrote without any other things like lighting a candle etc. made maybe my intent already clear. So at that night, when I performed it, I didn't feel anything at all and so I laid back and slept. The next day however, while focusing and putting something "meditative" on, I started feeling sensations. So obviously the ritual did work. At the very beginning, these sensations were really very lightly, like a light that burns with 0.1 Watt and sometimes switches back and forth to 0.0 Watt, as if not getting enough power. Yet, things became hour by hour more and more intense. Still doubting though at that time, if it's just my imagination and wishful thinking which made me contribute every little thing I paid attention to, to this. But it was soon all so clear, that by around the 2nd day I stopped doubting that it could been just my imagination. From that point, things increased with a big intensity, every some hours had new sensations and more. She kept her hands almost all the time on me, that was quite distracting hehe. So, 4-5 days forward, I were feeling clearly fingers and hands, one night I could even feel clearly someone as if I would touch something and the other could also interact with me in ways that allowed other practices, bdsm as example as there were feelings like binding arms together like some kind of energy string around the wrists, a "ghost" hand holding an arm into position or pressing it on the body (not just simply the pressure many describe, a lot more defined) etc., also the other could apply like energy outside and below a relaxed arm to move it into another position and such. I even found a method of communication at that time.
I suppose, as I've been completely open at that time and allowing it all to come through and having no blockages, progress were extremely fast possible. At that point, things took a turn however. Things happening, like when taking a nap on my relax chair, I woke up with what felt an energy field surrounding the whole body and when getting up like having shaky body control until I "shaked" that field/energy off, speak until it vanished. When trying to work on the computer, someone getting in the way and pushing my fingers aside (which made for some wrong keys pressed) or from below, like not wanting to let me type properly. I started feeling sick, going 10+ times to toilet for no apparent reason all day alongside feeling like some energy doing something in my digestive system. I wouldn't sleep at night. Toilet paper sticking to my fingers as if it were static. And motor skills like when walking or moving hands affected, as if someone would interfer with it - as example, me starting a movement and someone finishing it or assisting into doing it faster or some like that. At night I did hear someone laughing on two occassions or so, not just in my mind, but with my ears as I could distinguish the direction and distance it were coming from, so I suppose my astral senses opened too. I weren't sure if it were a laugh of someone just doing a prank or worse. Needless to say, there were 2 days and nights that became quite scary at this point and me in a really bad state. So I searched for a way to make it all stop alltogether, which I found and it worked within of 1-2 hours of events that felt like my body being cleansed part by part, things removed from inside (through kinda forced breath out) and after that some never felt peace as if I could take on the whole world and need to not worry about anything and at the end hearing a voice in my mind saying "relax now". (the details of these might be things many won't believe as also too many, so I'm leaving all for another time).
So, after this story it should be more than clear, that the letter method is not without dangers. From all I know today, being aware of some things and protection is to some extent important, as also to know exactly what one is doing before performing such a method, no matter what some (or Ryan) might say. As it is also important to know what you can do, if things go totally wrong.
The letter method first of all is like a contract you make (it also is without doubt a ritual, as there's an offering you make and a procedure like burning something). You may write things in there, that seem absolutely clear to you at that point, but realize later, that you needed to be more precise and detailed. You may need to limit things to time specifically and in general. And you may need to be aware, that depending on what and how you write things, you may actually do kind of a pact here. You may need to even include things like that you are going to put house rules up that must be followed. Don't leave yourself vulnerable or open to things that you might not be aware yet, how things might affect you or your life, because you focus too much on desires.
I closed the ritual properly, there's no mistake here. Yet, something came through (probably at a later time) that affected me. Recently I read, there are astral parasites that can get through the 3rd eye or such when one's abilities increase and the effects mentioned with them sound similar to some of what I experienced (as example digestive issues). On the other side, some effects might have been because maybe I couldn't been able to adjust as quickly to such an intensity I became open in less a week... where others seem to reach such states only after weeks, months or years. Others however might have been a malevolent entity or spirit. I have to say, I also quite pushed it, taking every opportunity for increasing awareness, which seemed just ok at that time.
However, do not think, that you are aware of all that could happen to you and are ready for all, especially if you are just new to all this! I have learned so much in the last 4 months and yet still know too little of anything.
I suggest, you don't use neither blood nor semen for the ritual. I used semen, supposedly less potential than blood even. As was said elsewhere, blood leads almost always to haunting or worse. Better even, don't use any ritual at all... and as you will find in the questions/answers post from the collective, such is not a necessity. If intent is of most importance as even stated in the letter method, then this surely can be expressed by other non-ritual ways.
Today, I'm still involved with succubi, however, this time they came to me without any ritual 2-3 weeks after these events. The progress has been also more gradual, more like by the week vs by the hour as previously and I haven't reached the state yet I had reached back then in less a week. I suppose, I'm probably also more careful this time and maybe there are blockages this time because of the previous events. I'm also not able to communicate both ways, which I were shortly back then. Still I believe it's a fast progress.
That's my advice to you. After all, if you are destined for a succubus and if it's like the questions/answers thread says, that they just wait for your first step, then it's going to happen anyway. If you are not, the letter method won't do it either (as there have been people too not achieving with it anything).