There are a number of tricks to using a router. First, the bit spins in a clockwise direction, as you look down at the router from the top. This means that when you cut with the router from left to right, the bit will tend to pull the router away from you, and when you route from right to left, the router will pull towards you. So, if you're hooking the edge guide along the near side of the board, route from left to right, and when you're hooking it along the far side of the board, route from right to left. Second, always test the position of your bit on scrap material. Your odds of getting it exactly right by eye are nil. Third, don't cut more than 1/4" deep on a single pass. We want a 3/8" deep grove, so make your first pass at 3/16", or thereabouts, and make a second pass to reach the full depth.
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.)
Grizzly is a company currently undergoing an upheaval. This brand was once known as a manufacturer of professional grade tools and equipment for woodworking products. Though, in recent years, that reputation has begun to slip a bit since being bought by the Chinese company SIEG. These days, whether the Grizzly product stands up to its previous standards of excellence often has more to do with what you are buying than the company itself.