The way I've made these, by the way, has them screwed securely onto the wall, they don't hang on a hook.
Step 1: Materials
Sheet of metal- Must be steel! Aluminum is not magnetic. I got 26ga plated steel, already cut to 24"x24". I've done this with other sizes as well, so get what suits your design. This piece was $15 at the local Ace Hardware. To me the best way to do this was to figure out how to make the pre-cut sizes (or multiples of smaller sizes) work for your needs; that seemed easier to me than cutting the metal to some other size.
Decorative fabric to cover the metal- Just get a simple thin cotton fabric. You don't want too much bulk or else your magnets might not be strong enough to hold much. Make sure you have a piece that's at least a few inches bigger all the way around. In my case I had a piece that was about 33" square, giving me 4-1/2 inches on each side to wrap around to the back. Since this is going to have a frame, you could actually use a piece that JUST covers the front side of the metal- start with only a bit extra & trim around the edges.
Balsawood strips- Since the frame isn't structural, just decorative, these balsa strips worked well. They are nice & thin, so they don't add a thick frame around the board. For my frames I got the thinnest they had, 3/32". Each side needs to be cut as long as the metal is, but the store didn't have 24" so I got 36" & cut them shorter. And you will need two pieces of the desired width per side. Whatever thickness wood you get, the sides of the frame will be double that thickness.
Paint- The regular craft paint works well. Covers in one coat & dries quickly. Of course you need brushes for it as well. The way my kids wanted the frames was to have two colors, one on the verticals & one on the horizontals. Of course it could be all one color, 4 different colors, whatever you like.
Spray Adhesive- I used the Elmer's brand, but anything with good adhesion is fine.
Iron- Need to get the fabric nice & smooth before you glue it on
Drill with drill bits & screw bit
Hot glue & gun
Four Screws- I really like the ones with a round head and built-in washer. The washer is good to really grab onto the board as you mount it on the wall, and they are a little nicer looking, make more of a decorative statement. But it's up to you what variety you use. Just make sure it's long enough to grab on the wall, at least 1".
Plastic wall anchors- unless you're attaching this into studs or some other piece of wood, you'll need to use anchors to get it to securely mount onto the wall.
Hammer or similar
Step 2: Cut the Wood for the Frame
Also, they need to be cut to the length of the sides. My metal sheet was 24" square so I cut all of the pieces to 24" in length. If I was doing a 12"x18" sheet of metal, I'd need two pairs 12" long and two pairs 18" long. You get the idea. To cut the length I just used a craft knife & metal ruler.
Step 3: Glue the Pairs to Make Each Side of the Frame
Step 4: Mark, Drill & Smooth Each of the Metal Sheet's Corners.
Step 5: Paint Wood Strips & Glue Together Into a Frame
Paint the edges & both sides of the wooden pieces. If the paint requires two coats to fully cover, just do that on the edges & one side. The other side won't really be seen, but it's still nice to have it painted in case a bit of it shows.
Once the paint is dry, setup the pieces to be glued together. The way these pieces are made the strips overlap each other at the corners, alternating around the square which is on top (or you can have both verticals on top, or both horizontals, whatever you want). Lay them into place & then check to make sure it's as square as possible. Without bonking anything out of whack lift up the top strip at each corner, one at a time, and glue the wood strips together where they overlap. Be sure to clean up any excess glue, especially on the side that will face out. The underside can be cleaned up after it dries if needed.
Step 6: Glue the Fabric Onto the Metal
Have your fabric smoothed out neatly on a firm surface, wrong side up. You want to be sure it's at least pretty well ironed so you don't end up with wrinkles glued in place. I noted lightly in pen in the two corners closest to me where the metal sheet should land when I lay it down. If you have lots of extra fabric to fold to the back then maybe it's not as critical how evenly you lay the metal sheet down, but I wanted to be sure I didn't end up with a big bunch of extra on one side & not enough on the other side. Also, if there are stripes or some other directional print in your fabric you want to be sure it's not crooked. The spray adhesive I used said that for a permanent bond it needed to be put into place within 15-20 seconds, so there isn't much time for figuring out placement & such.
Put out enough scrap paper to cover a space a good bit larger than your metal sheet, lay the sheet down good-side up (mine had a price sticker on one side, I wanted that on the back side), and spray a nice even coating of adhesive. Now quickly pick it up at the edges, flip it over, and place it down on the fabric. Press it down a bit, then quickly & carefully flip it all over so you can smooth the fabric down on the metal sheet.
Now take the who thing back over to your scrap paper, and for one side at a time spray the fabric with the adhesive & fold it down onto the back of the metal sheet, adhering the fabric to itself off the edges. I like to do two opposing sides first, then the other two opposing sides. Make sure as you fold it down that you pull it snug up against the edge & don't get a bubble along the edge. Also, particularly if you are not doing one with a frame, make sure that when you fold down the second two sides that the folded edges stay within the edge of the metal sheet & don't stick out. It can be a bit lumpy on the back, but you don't want it to stick out & show.
By the way, you might want to wear thin gloves for this step, your fingers can get quite sticky!
Step 7: Place Fabric-covered Metal on Wood Frame & Glue Into Place
Once it's nicely centered you'll want to glue it in place with hot glue. You could also use duct tape or some such method if you don't have hot glue, but I think hot glue works best. Basically you just want to tack the two together to make it easier to install it. Carefully lift up the metal one corner at a time & put a bit of glue on the wood, then press the metal firmly back down in place. Do this along the middle of each side as well. Do corner, side, corner, side, etc. As you pull up each corner or side try not to actually bend the metal. Try not to glue under where the hole was drilled in the metal, as that will make it harder to get the screw through there.
Step 8: Transfer Hole Through the Wood
Just poke through enough to find the hole on the other side. Once all four are done, flip it over & improve the hole a bit from that side. That reduces the risk of flaking a bunch of your painted wood off. Another reason, though, that it's good to have the washers under the screw heads- covers imperfections. ;-)
Step 9: Get the Wall & the Board Ready to Mount
Take the board down (good idea to remember which side is the top) & you should have four holes in the wall. If you're using the plastic wall anchors, drill holes now where each of the awl marks are, and insert the anchors. If you're not using anchors, pre-drill holes in the wall.
Take each screw one at a time & position it in a hole in the wood frame. Using the drill screw it in through the wood, slowly, and use your fingers to hold the fabric on the underside firmly such that it doesn't swirl around as the screw turns. You want it to go THROUGH the fabric and not push the fabric out or twist it around. Once it's all the way through, back it out at least half way. You could trim some of the fabric off the back around the hole, but if you work carefully it's not a big problem. When they are all in place, line the screw tips up with the anchors in the wall, top ones first, and start to screw them in a bit. Once they're all lined up, screw them all the way in. The balsa is VERY soft, though, so be sure not to go in too far!