What are the steps for m9 magazine reassembly?

Answer 1

It should have only been disassembled at operator level. You place the barrel into the slide, place the return spring into the slide, mount the slide onto the frame (pulling back the slide into place), flip the slide release up, then perform your functions check.

Answer 2

Insert magazine follower into top coil of magazine spring. Turn magazine body upside down, and place follower and spring in the magazine body. Attach floorplate retainer to the magazine spring, and center it. Compress and hold spring and floorplate retainer into magazine body. Slide floorplate onto magazine body until it clicks and latches in place. This and more information can be found in TM 9-1005-317-10.