Rocket Surgery Made Easy written by Steve Krug.
It's probably the field guide to user testing digital services. It's a three hour, ish, read filled with advice for everything usertest.
This is a short summary. Plus my own experience.
Rocket Surgery Made Easy
Watch people try to use what you build. Ask them to think out loud.
No matter the experience you have, people will always find ways to do things the way you didn't intend. This is why you test what you make.
You get perspectives from people that aren't you or your team. Get as much variety of people as possible to test your product or service.
Test early, and often.
Avoid going to the same people more than necessary.
Give the person some incentive for testing. Don't talk about the incentive until the test is done.
From my experience
Use your own channels to recruit people.
Use language that is simple to understand.
Be specific with requirements. Like what type of device the test is for.
Ask if there are needs, like assistive technology, required to complete the test.
Double check your testscript and setup.
Ask open questions, or follow up.
Don't ask closed questions, or lead.