First I have to say this is a dangerous practice, there's bacteria that grows on the cotton, and it can cause cotton fever.
Methamp is easily water soluble.. so just throw your cottons in water, use youre needle cap to mash the meth out of the cottons, draw up through new cotton..
I really don't condone doing this though, you shouldn't have any cottons lying around, because you should have been using a micron filter.