Picture of How to assemble a skateboard
This instructable will teach you how to change a skateboard and or replace the trucks. The process is fairly easy and straight forward. It is most convenient to assemble a skateboard on a work bench such as the ones found at TechShop.
Materials needed:

#2 philips screwdriver
3/8 wrench 
Remove these adsRemove these ads by Signing Up

Step 1:

Picture of
First, with a 3/8 wrench and phillips screwdriver, begin to disassemble your old skateboard (disregard this step if you are setting up a completely new set up). It is important to mark which truck is your back truck and which is your front tuck before removing both from your previous set up. If the trucks are misplaced on the new skateboard, the board will feel as if it is backwards due to the adjusted bushings from the previous set up. 

Step 2:

Picture of
Now, begin to insert the screws into the new deck. Hardware can be reused, but new hardware is recommended. The problem with reused hardware is that the screws can become stripped after excessive use, or the nylock can be weakened. With the screws in, place the back truck on the tale side and the front truck on the nose side. 

Step 3:

For the last step, place the nylocks on the screws finger tight. Once all nylocks are finger tight, begin to tighten them with the wrench and screwdriver. When tightening, hold the screw in place with the screwdriver and use the wrench to do the turning. This will help the screws sink into the board without twisting up the griptape at the head of the screw. It is important to tighten each screw evenly in order to prevent any one side of the truck to be more tight than the other side. If the hardware is tighter on one side, the screw can break under impact. Lastly, sink the screw into the board until it is flush with the griptape. And that is how you change a skateboard and or trucks. I assembled my new skateboard at TechShop.
I apologize for the comment about the simplicity
Now now, he does make a good point with keeping the back and front trucks straight that most people don't think of. Also, shock pads? I've never liked putting trucks right onto the deck, especially on something where there's a lot of impact at the trucks like on a street deck.
Oh I totally forgot to mention shock pads. I never use them but I know there are many out there that do. Sorry about that.