Getting the floor plate to lock into place. The two tabs on each side are really stiff and difficult to get into the magazine body.
You need to ensure that the locking plate is correctly oriented; it is not unidirectional. If you turned it around, it will not fit correctly in relation to the floor plate.
Once that is done, slide all the parts into the magazine body (except the floor plate) and hold in place with your thumb.
Then grab the floor plate and insert it at an angle so that one of the locking tabs is slightly higher inside the magazine body than the other.
Using a punch, come in from the bottom, through the hole in the floor plate and force the locking plate and magazine spring up into the magazine body, relieving pressure, and press up on the opposite locking tab on the floor plate.
If the floor plate and it's tabs are tight, try positioning the magazine on the edge of your work bench and forcing pressure down.