I sit forward on the toilet and dangle...
Here's what I have found from trial and error.
When using a 1/2 inch needle with Bimix or Trimix:
I stand and use the pump to expand the tissues, pumping up a couple times and letting the blood flow back out. This gives me a 'fluffier' or at least plumper penis. I already have the syringe and the alcohol pad ready. Sitting forward on the toilet, letting myself dangle, I grasp the glans, press the penis against my thigh, swab the side of the penis, allow a few seconds for the alcohol to dry, then inject somewhere between 1 inch from the body to 1 inch behind the glans at the 3 or 9 o'clock position. That doesn't leave a lot of area left.... After injecting, I switch my grip to the base of the penis and grip the penis to prevent any drug from running out into the body. I also gently massage the cavity portion of the penis, spreading the drug out into the cavities as much as possible. I do this for a minute or so, and keep the penis pinched down for a couple minutes before releasing. I allow a couple minutes for the drug to adsorb, then gently stroke the glans to stimulate the penis. This results in rapid enlargement, usually within 5 minutes. I wait for a full 30 minutes at least before attempting use. If use isn't right away, or is delayed by foreplay, when the hands aren't available for some continued stimulation, then the erection will usually subside a little. Some more stimulation will bring it right back to attention when it is needed. My drug induced erections usually last between 2 and 3 hours, and can be revived several times, if desired while it is slowly subsiding.
When using a 5/16 inch needle with Bimix or Trimix:
I do not pump up, but use the milking hand motion from the base to get a little blood into the penis, the rest of the process above is the same. A shorter needle needs less penis thickness to get to the center of the cavity is why the difference. If I don't pump when using the 1/2 inch needle, I tend to go all the way thru the cavitiy and inject in the center dividing tissue between the right and left cavities. When in there, the plunger will not go down. The material is so resistant, you have to pull back the needle to get back into the cavity to finish the injection.
I just counted up and divided and found my success rate for injections is 71.5%. Even with experimentation with different dose levels and considering a 50% or more erection as a success, there's still a close to 25% failure rate. What affects that is something I haven't figured out. I try my best to repeat my successful steps each time, so go figure. Anyway, probably more than you wanted to know....