I was reading about this on another board.
Put the stickers on the pieces BEFORE plugging them in. (I always recommend using a pair of tweezers in general, makes sticker application so much easier. I'm terrible with them too).
There's also apparently a tool included in the set to help you then plug the pieces in properly. So look for that, as a fair few people on the other board missed it and didn't realise that's what it was for.
Looking To Buy Either Siege Or Earthrise Ratchet, and Buzzworthy SS86 ER Cliffjumper.