Best/easiest way to install access door to dead space in house
I'd recommend a prehung knee-wall door. A prehung door is already on hinges in a door frame. A small 24 x 48 door would be very manageable. You can buy them online in various sizes. I understand you're not an experienced handyman but a small pre hung door would be a fairly easy installation for you and since the door is hidden , if it's not aesthetically perfect - so what? You've learned something.
First of all you have to determine if it's a load-bearing wall. If it isn't, cutting out one stud and framing in the door would be fairly easy and there are plenty of diy Youtube videos on how to do it. If it is a load bearing wall come back to SE with that as a separate question.