Best if you can get another set of hands when installing a large decal.
Get a spray bottle of soapy solution, one of those 3 x 4 plastic squeegee's ($4 off Amazon), a roll of 1/2 tape and micro fiber cloth.
Measure the placement of the decal and use the tape as your reference guide, one strip along the bottom and another to the right side. Something like the letter "L".
The decal is sandwiched between a backing paper (the bottom) and the transfer paper (the top or outer).
Once you've cleaned the door really well, hold the decal long way (top to bottom) and start at a corner of the decal and slowly pull away from one another separating the backing paper from the transfer paper (the outer) while spraying the decal with the soapy solution. Basically you're going to be spraying as you pull. This helps with the removal of the backing paper from the decal as well as the decal from sticking to itself if something was to happen such as wind. The transfer (outer) paper remains with the decal.
Once you've got the backing paper off, spray the entire door with a copious amount of the soapy solution as well as the decal once again. Use your second set of hands to help rotate the decal and place it on the door. I can't express this enough..don't be afraid of using your soapy solution on the door and decal. This helps with sliding the decal into it's correct position.
Once positioned to your liking, hold onto the decal and start working the solution out using the squeegee and micro fiber towel. As the solution is squeegeed out the decal will begin to stick to the door. After all the soapy solution has been worked out you will be able to pull (slowly) the transfer paper off. Blot dry and your done.
Just be patient, it's not that hard.