So, welcome to the forum, there’s a lot of reading ahead of you!
You have two major things to consider: joining your pieces and aligning your layers.
As for joining, here’s an exploration of some butt jointing techniques that I wrote up a while back:
And for alignment, I like to use registration pins, specifically brass rods, but you can use anything from dowels to pegs that you cut on your GF. I did a series of posts on mandalas too, they all use pin registration. You can find them here: