You want to drive the solution through the filter. Put the micron filter on your luer lock. Pop the back out. Put your drugs and sterile water in there. Then put the plunger back in. flip it and take off the filter. Remove all the air. Reattach the filter. Then push your solution (mix it up as best you can) through the luer lock micron filter. I always would suck from the micron filter into the syringe I was going to shoot with to avoid contamination. There are also guides on the Internet.
Keep in mind, you want to prime your filter with some of your water first. I always ended up running water though my micron filter until it dripped clean through. Then I would do the method I mentioned above and shoot up, but then I would feel like there were drugs left in the filter. I'm not sure what you're shooting. If it's black tar heroin... You want your final product to be light tan.
Or you can follow this video...
https://www.youtube.com/watch?v=fxnQsJTbhjY he's got a good method.