Describe it however you want. If you have a decent barber, they should understand you fine and work with you.
I’ve been shy, straight forward, and all manners of sides when asking for a haircut and the biggest issue has only been the occasional language barrier.

Once you’re comfortable in a style it’ll be easier to use different wording, depending on how you want your style. For example: you most certainly can ask for clipper sizes, like “no. 2 on sides”, however if you don’t know what you want from the top you could just ask them “a bit short but matching the sides, if you know what I mean?”

Some styles have different options, such as skin fades (usually medium or high here, indicating the point where the fade reaches on the side and back of the head). If you’re unsure definitely do not be afraid of asking, if any barber has an issue with working with you they aren’t the barbers you want as they’re likely to rush the haircut regardless of what you say (in my experience), or only know limited styles/lack broader experience in hairdressing.