When i first bought the brush and fluidline i tried the usual way of applying liquid eyeliners by dragging (can't think of a better word atm) the brush across but the line always ended up too thick and sorta uneven, so one day i just decided to do these tapping motions, sort of like how u read sometimes where ppl tell u to dot ur eyeliner and join the dots, but instead ur doing _ _ _ but overlapping just a little bit. When i apply my fluidlines like this, i end up with a really thin fine line which i love.
And i use the MAC brush cleaner, i try and wash my brush after each time i use them with my fluidlines, because i once left it overnight and it was abit more difficult to wash off. so now i just put some brush cleaner onto a tissue and wipe my brush on it until there's no trace of the fluidline on the brush and tissue. then at the end of the week, i wash the brush with baby shampoo.
I'm not sure if this is the correct way of applying it or washing the brush, but thats just how i do it. So hopefully it helps u too.