I've installed it on a Baritone Uke.
My best advice is inserting the X-brace leg 1 leg at a time until it's all the way through.
Take a lot of time with this. I did 3 or 4 practice runs before I really felt comfortable with the process.
I slid the top of the X-brace against the inside top of the guitar and then slowly lowered it into place once it was far enough back.
Since I couldn't fit my hand in, this was a 3 finger installation and the hardest I've ever done... good luck.