The YZ's typically have a screw that when you remove it opens a path to the transmission case. You fill until oil comes out this hole.
On my YZ's (86, 96 & 97) this screw is in an awkward spot, sort of behind the brake lever and to the side of the kick starter. It looks just like all the other screws but if you look closely you may notice that it is a little bit more inside than all the others and it may have a larger washer/gasket under the head.
I suggest that you buy a manual for your bike. It will probably show you a picture of where that screw is and answer a lot of other similar questions.
Rod