Every motor is a bit different. Your best bet would be purchase a service manual, or at least provide the make/year/model of your motor and hopeful someone here may have done an impeller job on the same motor before.
There's a really good video on youtube of a merc 150 impeller job. It is pretty similar on most outboards, and is really easy to do. Do it once and you'll never forget how to do it and never pay someone to do it for you again.
