It is really easy. Just copy a row that is already there and go insert copied cells. I usually do above the cell I copied by at least a cell or two. Then you just make the modifications.
Pretty straight forward once in there. just make sure if you do not have a Tier piece and the gear you copied was one to remove it from the settings. Also you need to make sure the slots (gem slots are correct if it has any.) Those are straightforward too. R,Y,B,M (red/yellow/blue/meta) Also for example (just an example) if your socket bonus is +6 dodge make sure to remove the use socket bonus "1" and put "0". This way it wont look for gems to use it. You might be better off with +32 AP in that blue slot instead of a blue one to get the +6 dodge.