Make a Speedcube

This article will explain how to make a great speedcube! Remember, though, that the cube is no greater than the cuber.

Steps

Learn that if the cube is a DIY kit, build the cube first to make sure everything works fine and the pieces are not defective. Leave off the center caps and stickers, for now.
Get the right kind of cube, the inside of the center cube should contain an adjustable screw and washer.
Use your screwdriver to adjust the tension the way you want it. Looser cubes have better tolerances and turn easier, but tend to fall apart or "pop". Tighter cubes don't turn as easy, but they are easier to control and don't pop. Once the tension is set, take one edge cubie out of the cube.
Spray the inside of the cube with Silicone Lube. This is important to make the turning much quicker and smoother. Afterwards, replace the cubie and wipe off excess lubricant from the outside. Before replacing the center caps, check the tension again. The turning speed will have changed since the lube was applied.
Replace the center caps.
Apply stickers. The official color scheme is (when white is front face): Front = White, Top = Green, Back = Yellow, Bottom = Blue, Left = Red, and Right = Orange
Finished.

When applying the stickers, it is best to use rubbing alcohol to clean the faces of the cube first. This removes any residual silicone and oils on your skin.
If the center caps don't come off easily, use a pocket knife, screwdriver or other sharp blade to wedge them off.
The only way to loosen a store bought cube is to sand down some of the parts, namely the underside of the center cubies, or to open up the edge and the 2 corners on top of the cube and spray silicon spray.

Making a cube very loose can or will cause pieces to pop out during solves.
Beware of using oil or petroleum based lubricants, as these can eat away at the plastic of your cube over time and just end up gunking up the inside of it.
Do not practice cubing/finger tricks on a tight cube, such as the Rubik's brand original one, in order to increase strength of the fingers. This can result in Rubik's Thumb or Rubik's Wrist, both documented medical conditions. If you want to increase finger strength, try using the speed cube instead.

A DIY cube or any adjustable cube (Studio Cube, etc.) NOT a store bought cube, as these aren't adjustable.
Silicone Lubricant
A screwdriver (Optional)
Sandpaper (Optional)