Instructions
Start a new level in "Garry's Mod" or load an old one.
Hold down the Q key to open the spawn menu. Click the "Objects" tab and select an object you want to spawn.
Repeat the previous step for all objects you want to weld into a prop.
Hold down the Q key again. Click "Weld" in the right-hand sidebar to equip the welding tool.
Point to one object and click it at the precise point you want to weld it. Click another object to weld it to that exact point. Repeat this step for all other objects to finish making your prop.