I just had a body shop do all the work since they were painting the pieces etc... But the SMZ lip fits right over the existing lip. But I believe on TT's the old front spoiler has to come off. However on my NA the SMZ front spoiler fitted right over the existing one.
When the pieces came from Stillen, I brought them out to my car and see how snug they were, and to see the fitting. The SMZ spoiler actually snapped right on and held itself in place. Very form fitting. I think it would be a moderately easy DIY.