Self-tapping Screws

[tico1028]

Hey guys, I just bought a rear diffuser (Vors type I) and front lip (JL Mot. GTS Style). Both arrived today, and both were bought from forum members here. Regarding installation, I wanted to know what size screws to use. I've searched the forum but haven't been able to find anything other than "use self-tapping screws and 3m tape." If anyone could help, I'd greatly appreciate it.

1) What size screws for the front and rear?

2) Are the screws currently in place on the car screws or bolts? That is, if they are bolts, they would have a nut on the other side that I should worry about; are they screws or bolts?

3) Lastly, Should I use double sided tape on both pieces?

Thanks.