I don't have any experience with using a double screen board. So, I can't help you there.
The easiest way to split would be just to split it in half(aka a 'walk away' split). You don't have to find the queen as long as you make sure that there are eggs in each half.
A split can be any size you want as long as there are either the queen or eggs and enough bees to raise a queen in each part of the split. If you have 4 boxes of bees you could split boxes 2+2, 3+1, 2+1+1, or 1+1+1+1. The ones without queen will make one.
How you split is determined by what you want to accomplish. Split alot to increase number of hives or just split a little to prevent swarming and produce honey.