To be honest working with small quantities & micron filters is a pain in the ass.
With that noted I've done it plenty of times.
Here's my quick take on it as I've only got a minute or 2.
Using a 3ML Luer Lock syringe I suck up 30-40 units of water.
Remove tip, replace with micron then push water into filter.
Next I pull the plunger back & suck the water back out.
I remove the filter, depress plunger, reattach & repeat.
I do this so that the 30-40 units doesn't dilute a 1ML shot.
Then depending on the pill one can either pre-filter in an oral syringe with cotton & suck it up.
Or if it's a smaller volume of pill matter you can just backload it.
Once you've got your solution in the rig (either pre-filtered & sucked up or backloaded either works)
just push down on the plunger till it reaches the bottom.
I usually push slowly so as not to blow out the micron as I've had that happen a few times.
Hold the plunger at the bottom for a bit & wait for it dribble on out.
With small volumes of water it takes quite some time to pass through the filter.
Sometimes I'll give it a good 3-5 minutes.
After doing so & letting the plunger re-rise, remove filter.
Be sure plunger is pulled all the way back for plenty of air & reattach.
Push plunger down again & let it dribble out again.
Doing so I usually don't lose to terribly much to the filter maybe 10 or 20 units, sometimes less.
Then if you so choose you can always rinse the filter for a 2nd weaker shot if so inclined.
Personally I find it really varies on what your filtering how much gets left behind.
If I'm filtering something that's a pain in the ass such as ER Morphine I'll often shoot a rinse.
Now if I'm filtering something like Oxycodone, chances are I'll just waterline the rinse.
Rather than do another shot for little reward, YMMV.
Hope that helps was gonna post in your micron thread but I was reading this at same time & saw it asked here as well.
Figured I might as well chime in. I guess this would be better in your other thread but I closed it.
Don't have the time at the moment to copy paste it over & you'll see it here anyways.
So I guess I'll just leave this answer in this thread for now.
Hope you find the information helpful & for all things micron filter related check out the link below.
Micron Filtering Mega Thread & FAQ