You'll find it's a bit of a meandering route. I've never gone from Port Credit to Scarborough in one go, but I've done each direction independently. Between Port Credit and the waterfront, you'll find the link-up to Queen's Quay is great. Do note that there is a missing segment of the trail from first street which requires you to ride on the Queenway. The return to the trail is unmarked, so you might just have to join it later...traffic is light and the curb lane is wide.
The next confusing bit is at Cheery Street. You'll want to cross Cherry Street and then get back to the trail a bit further away from the water as it meanders around some long-unfinished construction. It's then decent again pretty much all the way through the beaches. Be prepared for lots of clueless pedestrians in the beaches. No one ever looks before walking right onto the trail. I'd tell you to skip it and ride on Queen, but it's such a scenic stretch.
After you pass the water treatment plant, keep an eye out for small signs indicating the route. There's a pretty rough segment on Kingston road by the large church. Traffic is so unaccustomed to bikes there, that I recommend just using the sidewalk if you're on your own...it's Scarborough, so there's never anyone walking on it. The ride down and climb up from the Bluffs is lots of fun.