First things first you've got to find a factory building like below:
There are large factory areas that are pretty common, they are filled with crates which we will need to break for their materials.
After clearing out atleast one crate building you should have enough components to craft the recipe below (Police SMG) which we are going to be spam crafting for EXP as it gives a large amount and is a really cheap recipe. After crafting as many SMGs as possible you can disassemble them for some components back.
However Instead of just spam crafting it normally we can put our skill points into maxing Artisan for double the SMGs and Handyman for pretty much all our components back. This is abit overkill but it definitely helps speed up leveling if you're rushing for the level achievements or cant be bothered to farm all the crates.

After doing this for like 5-10 minutes you should be max level.