CHOOSING THE RIGHT SCREW FOR THE JOB
This is possibly one of the biggest skateboard ramp building tips I can give you. Use coarse drywall screws for indoor ramps and galvanized deck screws for outdoor ramps.
You're more than welcome to use whatever suits you best but remember, if you plan on taking apart that structure... nails simply won't cut it.
I've also seen some people swear by a particular brand of deck screw because of how well it lends itself to the construction of ramps. I have used this deck screw and yes it performs great but I don't think the results justify the cost of the screw. In my opinion coarse drywall screws are the only way to go.
If you're attaching plywood to plywood or plywood to 2x4's, 1 5/8" screws work best. If your attaching 2x4's to 2x4's, coping or transition sides to 2x4's, 2 1/2" screws work best.
Be extra careful when working with treated lumber though, the chemicals used to treat the wood contain a poisonous pesticide.
LUMBER
Dimensional lumber, like a 2x4 for example is not actually 2 by 4 inches. This is referred to as Nominal (what they call it) and Actual (what it is) size. Most of you probably know this, but for those who don't or would like a cheat sheet, here's a breakdown of dimensional lumber and it's actual dimensions.
Most dimensional lumber outside of North America differs from what is listed here.
WHERE TO GET YOUR SUPPLIES
The wood and hardware can be found at most home improvement stores. Sometimes you can find the steel there as well, I know my local Home Depot carries the coping and threshold material.
If not, you can search the internet for steel fabricators or salvage steel. Personally, I've begun buying my steel from MetalsDepot.com because of their very quick shipping and good prices. I'm not affiliated with them in any way, I just like them and their product.
If this ramp is going to stay outside, it needs to be protected from the elements. A good place to start is with pressure treated lumber, paint and a tarp. You may want to invest in a composite material for the surface too. Such as Skate Lite or Ramp Armor.
Be extra careful when working with treated lumber though, the chemicals used to treat the wood contain a poisonous pesticide.
SAFETY
I struggled with whether or not to include a brief entry on safety in this skateboard ramp building tips article and ended up thinking that it would be a good idea. However, I'm not your mom and I have no place to tell you to wear safety glasses or to be extra careful when working with power tools. Especially since I refuse to wear a helmet.
Though there is a difference between having confidence in your ability and taunting the laws of physics. Use common sense when you're building and you'll be a happier person for it.