I always found this method easiest.
Start by poping a rivnut into the dead center of the diagonal rail where the side and rear tub meet. Make sure its at the middle or lower, you dont want any rivets or nuts showing above the wheel arch it looks gay.
Clamp the rear tub in place and trim if nessasary. A few rivnuts on the rear diagonals should be plenty to hold it for now. You want to trim a dead straight line up the centre of the rail where the tub meets the side panels, then just use a small round file to make a half moon shape to allow the rivnut bolt to hold it down tight. Do both sides, then what i do is undo the tub where it meets the sides. Then slide side panels on behind the tub, you can then use a marker pen to mark the side panel cut using the tub as a ruler. This way even if your cut on the rear tub isnt quite perfect, the cut on the side panel will match exactly and youll end up with a perfect join. Hope that makes sense.
IIRC i took off the rollcage and covered the top of the tub in masking tape, then dropped the cage back down and marked around the mounting plates to get a precise line to cut to. Its on off on off to do it right and its also worth taking your time. Dont ever do it after an argument with the missus, it will end in disaster
One of those jobs you need to be really chilled out and in the mood for.