How many times do you have to return to the plumbing department of your hardware store before you finish that sink installation job?
It took me 5 trips for this job (in front of the TV cameras), as I slowly discovered things that didn't fit, or didn't reach, or I didn't think of ahead of time.
Does a professional plumber really do much better? Not really, he just has the advantage that he has a whole truck of parts sitting right in your driveway.
One suggestion is to lay out all the parts so you can visualise what you may be missing and check if everything fits together. In fact, you might want to do that right on the floor at the hardware store, it could save you a few trips back. If anyone complains, tell them I said it was OK to camp on the renovation centre floor.
By the way, that great double sink we were installing was made by Blanco Canada.