How to Create Cool Things in Garry's Mod

Get a general idea of what you want to make. It can be a trap to kill a trespasser, or a security device to allow only you in.

Spawn the necessary items for the device. More of the common props needed are the blast doors, bottles, and boxes.

Set them up with the physics gun. If you are creating a door that only you can open, set it against a wall so it's straight up and maybe move it into a corner to set it straight. You can also hold 'E' and press shift. This will cause the prop to align with a major axis, like straight up and down, or flat.

Attach thrusters, elastic, rope, etc to the props to give them the necessary effects. Some things you might want are doors that open with elastics and thrusters, so you would set the thrusters to a middle-thrust and the elastics to max strength, then when you press the thruster button it opens, but smacks shut killing anything in the way.

Add materials and color. The materials tool is really useful for giving it the final touches that make it look desirable. It also helps it to not look like a blast door in the middle of the map, but a cool looking force-field type door.

Make final touch ups. This may include things such as using breakables to auto-shut the door. This is done if the door somehow flips over and is stuck in the roof or something, set the door to a bottle with an elastic. When the bottle breaks, the elastic disappears and the door will try to auto-correct itself.