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.)

When you have the other leg seated, the threaded rods will extend father than you want them to. You'll want to mark them so they can be cut to length. Place a washer and a nut on each threaded rod, and then tighten down the nut to pull everything tight. Depending upon the wrench you are using, and how much longer the rod is than it needs to be, you may find it necessary to stack up a number of washers, so that the nut is positioned where the wrench can operate on it.
Lay a leg flat on your work surface, with the countersink side of the thru-holes down. Stick a piece of threaded rod in each hole. Take a stretcher that is marked to have one end adjoin the top of this leg, stick a dowel center in its dowel hole, line it up against the leg, using the threaded rod for positioning, You want the top of the stretcher to be even with the top of the leg, or just slightly above it. Give the end of the stretcher a whack with your rubber mallet. This will leave a mark indicating where the matching dowel hole in the leg needs to be drilled. Repeat with the lower stretcher than adjoins this leg. Then repeat for the other leg that will form this trestle, and the other ends of the two stretchers.
I’m building my second bench. I built the first years ago with wood from old machinery pallets – mostly oak and maple. Mortised the legs into the base, and pegged ’em with 1″ dowels. Of course, the legs warped a bit, but that made everything tighter and stronger. The top is 2″ thick Ash, one board cut in half and joined side to side. Wrapped a maple apron around the top, and I think I screwed the top from underneath to the leg braces. Solid. Over time, built a cabinet under for 3 drawers and a small cabinet door. Works for me.
I would say take the shed, you'll find some use for it. I have contemplated this question also. I have a 20' x 11' basement shop, thats quickly loosing space. If you live in an area with a harsh winters (say the Midwest), does it make sense to heat the shop through the week if you are only working in the shop on the weekends? Or do you run the heaters just when you are working in the shop? What kind of heaters are recommended (radient, wood stove, etc.)?
I made a template by scribing two adjoining squares on a piece of MDF, using compass and straightedge, then marking each corner with a centerpunch, then drilling the points with a 1/16" bit. I find I'm always breaking small bits, so I picked up a couple of each size some months ago, and on looking I found I had three 1/16" bits, which worked fine for what I intended.
With my second attempt at edge guides, I made the other rough-cut. On this, the edge guide worked fine, but the end of the cuts revealed that the saw blade wasn't quite square. So I adjusted the blade, clamped all four legs together, and made what would be the first final cut, if it came out clean enough. It didn't. I'd let the saw drift a bit away from the guide edge. So I adjusted the saw, moving the guide back half an inch, and tried again. the rough-cut parts were a couple of inches longer than they needed to be, so I had room to work with,. It's only the final cut at the other end that you only have one chance at.
Once you have all the clamps on, take off the scraps of hardboard. You can clean up the glue squeezeout with a damp rag.. When the glue is dry, trim down the strip flush with the panel using a router and a flush-trim bit. Then cut off the ends of the strip with a flush-cut saw, and clean up with a block plane, an edge scraper, or a sanding block. Leaving the ends in place while you route the edge helps support the router.

Out of all the shed books I have read, this book stays by my side while I hammer in the yard. The book is well written by what appears to be a non professional book writer (A Good Thing). It reads as if you are getting advise in laymans terms from your neighbor "The Craftsman", a welcome change from the DIY books, that assume a level of expertese. The level of detail is high and discusses gotcha's, advise, and trade "secrets" that could have only come from years of personal experience building sheds.