For plastic models like this, we recommend you use Plastic Glue. GF9 Plastic Glue is available in the online store...
Step 1. First begin assembly by glueing the wheels on.

Step 2. The longer of the two gun options is the 57mm gun for the ZiS-2; the shorter gun with the muzzle brake is the 76mm gun for the ZiS-3. Once you have chosen which gun you'd like, glue it onto the trail.

Step 3. The shield is attached directly to the gun. The bottom right picture shows where you glue. Don't forget to make sure it is facing the correct way!

Below. Correct shield placement.

Basing the crew

The artillery crew sprue has six figures, and each of your gun teams only requires five, so you will have enough options to give each gun team a different mix of figures. Position the figures around the gun as though they are busily operating their guns. Fill the spare hole using one of the plastic basing plugs. The ammunition boxes let you add some detail, making each base a miniature diorama.

Finished! Happy modeling! ~ George.