Above: You could use a compound saw or take it to your local hardware store to be cut to size, but I prefer the old fashioned way; a hand saw and a mitre box. It also allows you to make adjustments on the spot and be incredibly accurate.
Below: Each nail hole was pre-drilled and nails placed for ease of attachment. A tip - line up all your edging and only half nail in the centre nail for each side. This allows you to easily check placement, how they line up and allows you to make sure all the mitres align correctly. If not you can easily remove each piece and adjust it, then put it back on with the same nail. Whatever you do, DON'T put more than one nail in each side until you're sure it's a perfect fit.
Below: Brass corner brackets were added mostly for aesthetic purposes, but also because they protect and hold each corner together tightly.
The Final Product
It took three months of my spare time and more than a few curse words but here it is, complete and ready to hang. I may still change my mind about putting a clear lacquer on it, but for now I love it just as it is.
I'll be making a new tutorial soon for a custom piece I'm making for a friend. It'll be based on one of his (and my) favourite video game series, so keep an eye out. I hoped you enjoyed this tutorial as much as I'm enjoying my new map, all the best!