It's pretty straightforward. If you get ampules with powder and water, you break em open (careful about the non-scored ones, you'll want to use a scoring tool or metal file to do that otherwise they'll shatter) then use a syringe to retrieve the bacteriostatic water and inject it into the powder amp. Do this slowly to avoid frothy bubbles that won't rejoin the solution -- maybe harmless but it bugs me, seems like it won't mix evenly. Roll the amp or tap lightly to mix if needed, although it should dissolve immediately with no help. Once done with that, pull the amount you need back into the pin. If it's a 1500 IU amp and you need e.g. just 500 you'll want to refrigerate the remainder to avoid spoilage.
You can do IM or SQ on the pins. I went with IM at first since it appeared to be the recommended method but just 0.5 cc left far more pip than even 2 cc of mlg test-e, so I switched to SQ and it's much smoother.
