This is how I do it for 8x10. I am using a glass (not a contact frame) and I start the exposure immediatly after the glass is lowered. The Newton rings move around a little before they settle in to a fixed pattern. So, if the exposure is in the 30 sec range the rings don't show up or are minimized considerably.
You should be able to see the rings by looking at the reflecton of the safelight on the film under the glass. Watch how they move around as you raise and lower the glass and you will see what I am talking about.