Golden tip but I would add a thing or two.
Most probably both triggers R1&R2 will stay in lower half of the pad case with springs hanging there.
You must take them of their lower case sockets before assembly and fit them bellow L1&L2
When you feel click when fitting them and spring works like on normal assembled pad you're done.
If thats not the case do not try to assemble lower casing half as triggers will not work properly.