Assembling the parts
If you got this far, you probably already know how to assemble most of the parts. But here are some advice that might help.
When in doubt, search
I know it's exciting to put your board together, but it's not the time to rush and overlook something, or not doing it properly.
They are esk8 communities out there that will be able to help you if you have a doubt. There's a big chance that your question has already been answered somewhere.
Electric shorts ? No thanks !
It's a bit late to check that, but it's not too late ! Given the energy store in an electric skateboard battery, you absolutely want to avoid having a short somewhere.
Use a simple multimeter to check that electricity can flow where it should, and that it can't go where it shouldn't.
Use heat shrink tubing when possible to protect connections and solders. Electrical tape also works well too.
Vibration will make your bolts come loose
First advice is more of a warning: the vibrations your board will have to endure will make any bolt or nuts come loose. It's just physics.
You probably have seen nyloc nuts before, it's those nuts that have a plastic (well, nylon) part at the end of it. That plastic part is helping the nut to stay tight in different kind of situations.
Thread locker is also well known in the DIY world, you probably know the most famous brand that produces it: Loctite. Without going into details, Loctite offers 2 liquids: a blue one and a red one. The red one is the most powerful one, which is meant for bolts and nuts that should stay tightened forever. The blue is the most common one, it's making sure the nut or bolt won't come loose, but they still are removable if need be.
Of all that text, remember one thing : USE LOCTITE EVERYWHERE, especially when there's no nyloc nut to help maintain everything together. Loctite takes a bit of time to dry off and be effective, so don't assemble everything and go ride, wait for the Loctite to dry. Tight down all the screws and bolts the best you can.
You should also check your board before going out riding, make sure everything is still in place and tight.
Make sure everything is aligned
Check your wheels. If a bearing isn't properly inserted, your wheel can be slanted on the truck and turn weirdly/wobble.
Check your motor mount, it should be perfectly perpendicular to the truck hanger. If not, the motor, its shaft and the pulley that goes on the shaft won't be parallel to the wheel and its pulley.
Check your wheel pulley, it must be aligned properly with your wheel. Hint: put the wheel and pulley on a truck axle, make the wheel turn, look at the pulley to make sure it rotates without any wobble.
Double-check your electrical connections
Again, those damn vibrations can make two electrical connectors come lose.
The most common example is the motor bullet connectors. They aren't usually super tight and can easily disconnect themselves. Hint: use heatshrink tube over the 2 parts of the connector once they are connected, electrical tape works too.
Make sure connectors are connected all the way in. XT90 connectors have a long stroke length, check if they're correctly plugged.
Do some testing in a "safe" area
Got everything setup ? Now it's time to go out and try it out !
Remember, wear safety equipment. I know it's annoying and all. But please, don't die !
Try to find a nice and quiet area to test your board, like an empty parking lot. Make sure somebody knows where you are, you never know what could happen, some bards are really fast, and an accident can easily happen, especially when you're not experienced.