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If its PU, leave it in the sun for a few ours for it soften up and regain its shape. If fiberglass, then feel free to test fit right away.
Line up lip with a buddy, and make sure everything is snug. Afterwards apply 3m tape to lip, and then drill some self taping screws to secure it onto bumper. Thats the basic process
Alternatively, take it to a bodyshop for professional fitment
Also before you get it painted, test fit it to see if the lines match up etc. if not have the body shop make the proper and required adjustments before paint.