Today I just finished my buckler for rapier fighting. They are very simple to make, so here is how.
First find plywood. I used some scrap 3/4 inch AC plywood. 1/2 inch would also work, but no less than 1/2 inch plywood. You could use some interior VC plywood if you want it to look nicer. I then used a band-saw to cut it into a circle. I am also making one for a friend who wanted a diamond. Just use what you have to cut it. If you buy it at a lumber yard you may be able to ask them if they can cut it for you. It never hurts to try. I then used a bit of wood glue (any glue should work) and I put it in any cracks in the plywood. AC is an exterior wood that is usually not seen, so it doesn't have to look good, but it is still strong. The glue will help make sure there are no problems with the buckler. Once the glue had dried I used craft paint to paint it. My device is not yet registered, so I didn't want to put that on it just yet. Once that was dry I attached my handle to the back. I bought a zinc plated drawer pull from my local hardware store and it looks like it will work out just fine. You may need to get different screws to use though. The ones in my package were too long, so I found a few shorter ones and used them. You also need to put something around the edge in case of splintering. I plan to use electrical tape.
Now let me get a little historical on things about bucklers. Historically bucklers are metal and are circles, with an area in the middle that extends outward, like the one below:

In the back there would be a handle across where the dent it, which is where you would hold it. Bucklers are not meant to be work on the arm as a shield is. They are meant to be held in the fist so that they are lighter and faster than a shield. They are usually no larger than 24 inches in diameter, though I have heard of people using larger ones. They can also be very small, I have seen a few that are less than 1 foot in diameter. As I said earlier, they are usually round and made of metal, but they were made of wood sometimes. They can also be in shapes other than a circle.
Now lets move to a new subject. I have been needing some garb to fight in, and I just recently made myself a tunic that I will use for fighting. I have a shirt, but I am not comfortable using it for fighting since some seams are hand stitched due to the difficulty of getting a sewing machine to do the work. All I did was make the basic short sleeved T-tunic and then add sleeves. The t-tunic is not hard to make and instructions are easily found so I will not add them here. The sleeves were a little bit trickier. What I did was measure the length of my arm from my shoulder and then have the other dimensions be one foot, so that I would have a nice poofy sleeve. Then I folded it in half the long way and sewed up the seam. Then I pined it inside my sleeve in my tunic and sewed it up. I forgot to take into account that if you sew below the shoulder, you don't need to measure all the way to the shoulder... so I ended up with really long sleeves, though they are manageable. I did use a bit of elastic to make some bands to hold the extra fabric just above the wrist. I also put in draw strings at the cuffs with a thumb loop since I will be using this for fighting and it will keep the cuff from being pulled out of my glove.