First and very important

Clean the area thoroughly. use water mix with soap, DO NOT use any cleaners that have wax in them,  Measure and place the sticker in the desired location. Put a piece of masking tape on the sticker to hold it in place where you want it and to act like a hinge

 
To apply your sticker place a piece of masking tape half on and half off the top of the sticker.
 
Align the sticker on the surface and step back to sure that it's where you want it, and using a squeegee smooth out the masking tape to make sure it stays put.
 
When you have it positioned correctly lift up the bottom of the sticker and remove the paper backing from the transfer tape; caution, do not release the transfer tape.
 
Holding the transfer tape with one hand and using the other hand rub over the sticker with a squeegee starting at the top middle rubbing down the center of the sticker and then rub to each side of the sticker using firm pressure.
 
Rub Left.
 
Rub Right.
After rubbing the sticker down peel back the transfer tape making sure the corners of the sticker are not lifting up
NOTE, if a part of the sticker starts to lift up lower The transfer tape and rub over it again then continue to peel.

It is that easy, so why dont you go shopping for some decals and stickers