First, cut one long edge. Second, cut a short edge, making sure it's square to the long edge you just cut. Clamp both pieces of the edging you'll be using along the long edge you've cut, and measure the width of the base plus 1/4-1/2", mark that, and then lay out a line through the mark that is square to the end you've cut, then cut along the line. Finally, cut the remaining short edge square to both long edges. (The length of the top doesn't need to precisely match anything, so we don't need to bother with clamping the trim before measuring.)
What I did, when I came back, was to clamp down the strip where it had torn away, and then to start routing from the other end. I still moved the router from right to left, but I did it in six-inch sections, taking light passes, and sort of whittled the strip flush. As the sections I was working were farther to the right, the strip was thinner. Eventually I came to where I was trimming the strip away entirely, at which point I took off the clamps and the remainder fell away.
My first shop was a 8 x 10 ft room, actually a bedroom, with 10 ft high ceilings in a older home in Chicago. I had a 10" Craftsman TS, drill press, 8 1/4" Skil saw which still works, some welding tanks, hand tools and... my bed was a loft across the 10 ft length 4 ft down from the ceiling with a ladder against the wall to get up. I was 16 yrs old then, now I am 66 yrs, have 2 shops with Craftsman power tools up stairs, and Grizzly tools ,a Powermatic 68, a MIG welder, and other tools on the ground floor. I've come a long way, You think? I remember making bases for sculptures called "plinths", my first commission, in the living room, 5 pieces rather large, several ft long and wide. The house was haunted, and my mom told the "spirit" to leave and go back downstairs. There was a cold draft and the straw chair would rustle with no one sitting in it. Wow, does that bring back memories from the '60's.
I agree that even at 12x12, things'll be "cozy" inside. That's OK. Keep in mind that this will be a general purpose hobby shed, not just a wood working shed. Even 12x8 would be plenty big enough for building 1 meter wingspan bungee launched RC gliders. And for the electronics projects I do, I only need enough workbench space to do some soldering with a 3rd hand (for electric guitar pedals). If I need more room than the shed offers for wood working projects, I'll just roll the table saw down a ramp and do my cutting outside in the sun.