Stay with it, whisking the whole time. I know this seems hard to do but it is so worth it. You can do my wooden spoon trick like I use making gravy. Get a little on the back of the spoon, run your finger down the middle and make sure to turn the spoon to make sure custard does not run. Perfect test every time. You can also test your temperature to make sure it is somewhere between 175 and 180 degrees internal temp to be sure.