As I'm sure you've heard steelhead fishing is all about finding the fish. I'm going to assume you've likely fished for steel on gear so at least you know where to start looking.
The biggest mistake I see most spey guys doing is paying too much attention to their line.
Because we spend so much time managing line, and setting it up for our casts, we have a tendency to concentrate on it during the fishing part as well. Often we throw out a nice long cast and marvel at it as it swings through the likely holding water.
Instead, pay attention to what your FLY is doing. Don't just cast and swing, cast and swing. A rapidly swung fly is incredibly fast and constant in it's speed. It doesn't move or act like most naturals. It can catch fish but usually only attracts the most aggressive ones.
Cast to a likely line and set up a proper drift allowing your fly to get to the right depth and into the zone. At the end allow it to swing a bit but start adding some strips to impart a bit more motion. Work on mending, staying in contact with the fly, and proper presentation.
Think about things like; what profile is my fly showing to a fish downstream? is it the profile i want? can i detect a take right now? What depth am I at? Is my fly ahead of my line or my line getting ahead of the fly? Where will my drift end before the fly starts to race off in a swing?
Just like single hand flyfishing, it's about presenting as natural a fly as possible. Don't spend too much time worrying about fly choice. Steelhead will hit a giant 8" florescent pink worm, so don't let the fly occupy your doubts. Often it's the ugliest of flies that catch the fish. Work on good presentation and the fish will come.