Couple ways to do it, you buy what is called sterile oil or cutting oil (sterilized oil).
You can either pull in 1/2cc of each, then pull in 1cc or so of air, and turn the pin over and over letting the air bubble mix the goods up a bit. Or buy a sterile vial with a stopper on it already (or buy the full crimper kit etc), and then mix it all together.
I like to have sterile vials around to mix my stuff up ahead of time when I am injecting two substances or cutting a large amount of high mg/ml test or whatever. PL does now offer though a low mg/ml line, some of the items are priced higher though.
This is also very handy for re-adjusting the potency of your gear in other than 50% denominations.
Like for example, 200mg/ml prop which I was just using (cut in half though, easy). If I wanted to shoot daily, but wanted 75mg/ml, I could take 5cc of this 200mg/ml substance (prop) and add another 8cc to it in my sterile vial (20cc or greater).
This would give me 13cc of substance.
5cc x 200mg = 1,000mg (1 gram)
1000mg/13cc total = 76.9mg/ml
Or 400mg (2cc) / 5cc total = 80mg/ml (2cc gear + 3cc sterile oil)
You have your total substance in suspension over a total volume of oil.
Now aren't you glad they made you take math?