1st= dig untill under untill you find a slime (waring dont dig strait underneif you)
2nd= build a giant room where you found that slime then build some channels.
3rd= put some signs in those channels first on the wall then in the middle then on the other side keep doing that
4th= pour some water in the channels and poor som water on a layour on top of your water to make the big slime drown
5th make a ladder strait up from where you found the slime
6th= slime can climb ladders whaller you have your slime farm

if this was to confusing (note: i couldnt evan understand myself) check out this videos.

http://www.youtube.com/watch?v=Xaq11N2BMZE

http://www.youtube.com/watch?v=iuLmlKpACXk