A game master screen, if you've never seen one before, looks like this:

The general idea purpose is to keep a GM's notes hidden from the players. A game master will often roll their dice behind the screen as well. It's a means of preserving the suspension of disbelief, maintaining an air of authority and mystery, and facilitating fun secrets and surprises. It's also a handy place to stick a bunch of quick reference rule sheets.

Here's a step by step guide on how to make one out of plywood.

• Wood. You want enough for four pieces, each one about 40cm long by 30cm high, at about 3mm - 8mm width. The exact dimensions depend on what you want. Plywood is perfect, and it's cheap. If you're looking to stain it, though, you might get better results from a different kind of wood.
• Superglue. Anything that works on both wood and metal.
• Hinges. Six. Two per joint, with three joints. You can buy little packets that come with the screws. You want something small. See the smallest they've got, and probably go one size up. You want something strong enough to keep it together, but not massive. The screws are going to poke out the other side no matter what, but ideally you still want something small and easy enough to file down.
• Screwdriver. Electric is ideal, but not everybody has one lying around. Just find something that matches the screws, and do your best.
• Saw. Again, electric is ideal, but I was perfectly fine just using a handsaw.
• Clamp. Something to hold the wood in place while you cut out the peices. You want it sturdy. Frankly it doesn't need to be a metal clamp, but whatever you use, you don't want your wood moving around at all. Can't emphasise enough.
• Sandpaper. Again, an electric sander is amazing, but you can do pretty well with just the paper on its own to be honest.
• File. For the screws. If you've got an electric sander you can probably get away with using that, though. I did. Just be careful.
• Gloves. For protection. You might think you won't need them, but trust me it's better. Otherwise you end up with heaps of little cuts all over your hands.
• Wood stain (optional). Up to you what color. I wanted mine dark and gothic, but the black I went with ended up almost purple. If I could do over, I'd probably get a deep reddish brown, almost black. You could also go a warm gold, or a light pink.
• Brush/sponge (optional). To brush on the stain.
• Wax/laquer (optional). If you want a protective layer on it at the end. Warning you now though, you probably want a well ventilated space during and after.