When installing J trim you never over lap the bottom piece. You overlap the top so water runs over the bottom instead of in it. Just cut the bottom piece the way they said and slide the top over it.
I’m not a pro but I have two pieces in front of me and I agree with them: the top piece goes inside of the bottom. This sounds wrong until you look at the pieces; top cut and inserted creates a proper shingle effect. However it is made better by not cutting the nailing flange-only cut the second bit so it keys properly into the bottom piece. If you have bottom insert into top any water on the inside behind that edge of siding actually gets in behind the lower part of the j channel.
