After completing your masters degree, you will need to do a Clinical Fellowship Year (CFY). You will not be licensed and certified until you complete this year (it’s actually 9 months) and pass the appropriate tests.
Finding a good CFY can be tricky. About 15 years ago I was contacted by an agency to supervise a student during her CFY. She was working for a school district and despite the fact that there were many SLPs in her district, none of them had the time to adequately supervise her. It should be noted that school based SLPs are often very, very busy.