A relatively straightforward acheivement to get.
First off, you need to find vehicle blueprints in order to build the custom vehicles. For most people, you will pick up a lot of these as you progress the game (look for the blue circle items on the map).
You will also spend a lot of time travelling around in vehicles (its the safest way to travel), so mix and match here to open up these vehicles in the garage.
Next find a garage on the map, and approach the workbench. Using the pause facility, scroll along the top to look at vehicle blueprints to work out what you can build. You then want to select 2 vehicles at the workbench required for the custom build so they appear in the garage.
Then you need to move one next to the other so they are almost touching, get out and they should shimmer grey as you approach them. Then hold RB and A to effectively create the custom vehicle.
Do this 5 times, and the acheivement is yours! I found i couldnt do this in one go, as the garage wouldn't continually allow the cars to appear in the garage, so create a custom, drive round for a bit, go to another garage, etc. It will come with time anyhow. Good luck!