this is what you do (very rough. extremely rough. but itll do.)
you will need a mortar and pestle (small ceramic) and probably some lactose (pretty much all the drug companies use lactose) as well as some digital scales (expensive ones)
when you grind powders up using a mortar and pestle, you only put a little in at once. multiple batches are always needed, unless youre only doing 4 or 5 caps worth (if youre making caps).
put a little of your drug powder in the mortar and pestle. grind to a fine powder. ideally you would use a seive afterwards, but screw it, where the hell are you guys going to get calibrated seives.
remove the resultant powder, put it somewhere.
put a little more drug powder in the mortar. repeat.
keep repeating until you hve ground up all your drug powder.
put some lactose in the mortar and pestle. grind it. take it out. put it somewhere.
repeat until, once again, you have ground up all your lactose in small batches.
to mix them together, dont just dump them all in there and mix. youll get a mixture as homogenous as rocky road.
weigh out all you powders in correct proportions. lets say you want a 5% w/w powder of your drug, and you have 5g of drug (already ground out as above). you then need 95g of lactose.
once this is mixed, clearly youll have a 5% mix of drug in lactose.
put a small amount of drug and lactose in the mortar. grind it up for ages, until it is mixed up very well. take it out. put it somewhere. repeat with fresh drug, fresh lactose. keep going until you have used up all the drug.
you should now have a small pile of mixed drug/lactose, and a big pile of as yet unused lactose. repeat the whole process, this time using the drug/lactose mix, and lactose.
do that over and over until all the lactose is used up.
thats it. youre done.
deciding how much lactose to use is common sense. what you want is a final concentration which, per serve, is neither too much or too little to consume in one go. you dont want to dilute it too much, or youll end up needing to eat 10grams of lactose to get a serve of drug. neither do you want it too concentrated, otherwise a small screw up in measuring out a serve will result in a huge difference in dose.
thats how a bodgy bastard back yarder would dilute their powders. going into more detail is pointless because people just dont follow instructions to the letter and will cut it down to this anyway.
any people out there who want more detail but who arent doing anything illegal post here and ill get back to you
cheers