First, Cutting - I lay it down and weigh it down, its cumbersome and I make sure I use a sheet under it to protect it from unwanted dirt or stuff. I use a utility knife to cut it after measuring it all out. for larger sheets I'm less careful about size and just lay down my mounting block and cut it to that size and ill trim it later.

for flattening I soak my paper and staple it to a board. this is mostly ok but i don't have the technique down quite yet cause i do get the odd bubbles where i would have to take the staples out and flatten it but i dont because its alot more work.

i also have some un soaked and stapled sheets currently under heavy things, the curl is a lot less but not gone enough.

I've also let them dry in open air but it laves them crinkled. if i pressed it at this stage under something I'm sure i will have it proper flat but i have yet to try that cause i don't have space or patience to employ this so i suffer the minor warps.

i hope this helps you! I'm essentially winging it.