Typically, I will dress and tuck my penis back using a gaff or panty girdle. I then go out dancing or read transgender erotica to build my internal arousal. Back home, I get out a vibrator and stimulate the head of the penis (glans) as well as the area around and behind. This is while I am still tucked back. Eventually I can build up enough excitement to come, and it can be mind-boggling. Since my glans is roughly where my clitoris will be post-op, and the tucked testicles help in transmitting the vibrations all-around, it feels very feminine. Since the cock is engorged but not hard, I don't have to "tune-out the penis-ness", or shape-shift as much in my fantasies.
Since I discovered this technique gradually, I can't recommend a sure-fire recipe yet. Sex therapists often teach people with disabilities on how to shift their erogenous zones, so what I am suggesting should be possible for others. Here is how I would teach myself this from scratch:
First, get a coil-type vibrator. This is the kind that plugs into the wall and has interchangeable heads. Use the one that looks like a cup and teach yourself to get off while you are erect by placing the cup over the glans. If you are really used to stroking the shaft, do that too, but try and shift your focus to vibrations rather than friction.
Get a gentle gaff or panty-girdle that you can use to tuck your cock back and hold your testicles in. You want it to be gentle as it takes a while to learn how to build up sexual stimulation without getting an erection. I use the "AFI Femme" that I bought at Carla's. Leave the locking chastity devices in the fantasy world! You will "flip over" a few times while you learn. If that happens, don't fret – you can either let the stimulation pass or go ahead and enjoy a conventional orgasm.
If you have not worked on learning to enjoy prostate stimulation, or anal self-stimulation yet, you may want to try learning that too. Your body is a much bigger sex organ than your penis! Read up on foreplay techniques and learn to stimulate your breasts, ears, legs, etc., the way women like it to be done. (Note to TG admirers: Trannies love this slow tease too!)
When you do have a "girlgasm", it will feel different. Perhaps less seminal fluid will come out. (A condom or bit of tissue will help catch it). If you let the arousal tension build in your body, the feeling will be more intense and last longer.
Sometimes I do a little "self bondage", so that I can bear against restraint while I build and come. (Again, be careful and make sure you can "get loose" quickly in an emergency.) If I can find a gaff that exposes the glans and the anus, I want some way to rim my "vagina and clit".
A girlgasm might be a way of simulating what sex would be like post-op. You are using the same nerve endings and tissue that you will have with your neo-vagina, in roughly the same places. Since I have not had that surgery yet, I don't really know.
I would be interested in knowing if others have learned how to do this or something similar. Please write and let me know if these instructions work for you, or tell me what does.