Ohh OK.
1. Clean up the roof properly.
2. Take a sprayer and fill it with soap water.
3. Spray it over the roof completely no portion should be left out.
4. Now comes the most difficult part as you are alone open up the sticker and spray on the side having the adhesive.
5. After doing this paste it on the roof and adjust it no cutting should be done now.
6. If there are any air bubbles slowly pull them towards the outside and remove them don't let the sticker move from its place.
7. Wait till the sticker sticks to the roof then start the cutting with the help of a cutter.
8. Last but not the least pat yourself that you have done a difficult DIY all by yourself.
I know you are eager to do it as DIY but to handle such a big sheet is a very difficult bro. The boy could do it easily as he had cut the sheet in two halves. I would once recommend you to get it done by some shop. In Delhi I got it for 800/- only.