How to create a collar in minecraft?

As in a collar for a dog? You don't, a wolf will automatically have a collar when you tame it with bones. You can change the colour of the collar by right clicking a tamed wolf with dye

If you mean a lead, which works it any passive animal, then do it like this
s= string
sl= slime ball
X= nothing

S S X
S SL X
X X S