Where are you based?
I attended Usability week ran by NNG a few years ago and one of the days I attended a course about running a usability test. This was a great starting point. I then followed this up with lots of research and reading on my own. When I attended usability week you could attend single days, and I think you still can. You could also look at the courses that they run outside of usability week. https://www.nngroup.com/training/
When I moved to my current company a year ago they were in the process of starting internal usability testing. I helped them set up an usability testing lab. I wrote a post about it here Internal Usability Testing Labs. We followed this up by getting in David Travis from User Focus to do 2 days training with us. There was a group of about 8 of us and we chose to have this based around the software Morae, which we use for recording and analysing our user testing. This course was great and by the end of the 2 days everyone knew how to write test scenarios, set up tests, facilitate, observe, and anaylse the results. We now have a team of people who can train other people internally on how to perform a usability test. The full list of courses available through user focus are available here http://www.userfocus.co.uk/training/curriculum.html
I would say a course is really helpful to get you started but most of my knowledge has come from reading articles and researching usability testing myself. I have a folder of research materials which I am constantly referring back to. I have this in our office so other people can read through too an dI have highlighted important bits.
NNG have some great reports and articles on usability testing. This is one of my favourites https://www.nngroup.com/reports/how-to-conduct-usability-studies/
Here is a list of all their reports https://www.nngroup.com/reports/ (some are free and well worth a read!)
User focus also have some great articles about usability testing.
Hope this helps. Let me know if you have any more questions 
