I am going to describe the basic process for both types of flats. Standard, soft flats are what I learned to build in college. Soft flats were traditionally used by scene painters, even after plywood came into being, because painting an actual landscape on canvas or muslin is much easier than doing it on wood. There are very few scene painters left, however. Technology and a savvier audience, require an ultra realistic set for most plays, now. This means that, when flats are used for interior walls on the professional stage, they generally have hard skins made of Luan, and are painted solid or wallpapered (just like in your house). For an exterior scene, real foliage is now often used, rather than a painting of foliage. Though hard flats are not as good for painters and are much heavier, they are easier to build, require fewer steps and are not as fragile as canvas.
The basic lumber used for building theatrical flats is 1x3 pine. In the building of "Hollywood" or "TV" flats, which are not "flat", but are built with their frames on edge, you can use any width desired- as this will serve as a return on the wall, which the audience will read as the 3dimensional thickness of it. To save money,however, use the 3" width, anyway, and add a deeper return only at doorways, windows, etc. where the audience will actually see it.
Rail- The top or bottom piece of the flat, which determines the width of the flat.
Stile- The side pieces of the flat, which, when added to the thickness of the rails, make up the height. The stiles are always positioned between the rails, so that the part that is on the floor, as the flat is standing, is one piece and won't tear apart as you slide it into place, upright.
Toggle- Also called toggle rail, this is the middle
support, which runs side to side and keeps the stiles stable and
square. On a really tall flat- more than 8 ft.- you will need
more than one. On a shorter flat the toggle is centered. If you
use two, divide the height into thirds, and place the toggles
at those points. The toggles are the width of the flat, minus
the width of the stiles, so that they will sit between the stiles.
Corner Blocks- These are the pieces that secure the corners of a standard flat, in the back. Use 1/4" plywood cut into 7"x7"x hypotenuse right triangles( cut 7" squares, then cut those in half diagonally). Be aware of the direction of the wood grain of the plywood. It is important that the grain go across the joint in the two pieces of lumber you are securing (perpendicular to the crack between), otherwise, the joint will be weak and will break if it gets pushed on. I am including some pages from Backstage Handbook, their dimensions are different from what I learned in school, but that's okay, use whichever you feel more comfortable with.
When building a TV flat, no corner blocks are used. Long screws are sent through the rails directly into the ends of the stiles to make a box. Use glue in the joint.
Keystones or Straps- These are the pieces that secure the middle supports of the flat (toggles). They used to actually be keystone shaped, nobody goes to that much trouble anymore. Now, they are the same width as your lumber- 2-3/4"by the length of your corner blocks. That way, you cut a bunch of strips at 7"(or 9") and some of them are cut into squares and then triangles, and the rest are cut into 2-3/4" widths. Keystones must be cut with the grain going lengthwise. This gives strength to the joint.
On a TV flat, it is common to use corner blocks instead of keystones, to secure the toggles. Turn the corner block around so that the hypotenuse is parallel to the stile, and the right angle is bisected by the toggle. Glue and screw through the stile into the toggle, and through the corner block into both. It is not necessary to inset this piece on a TV flat, as there is no place to inset it. See picture.
Standard Flats- To build a 4'x8' standard flat, the two rails will measure 4', the two stiles will measure 7' 6-1/2"(8' minus the 2-3/4" widths of the two stiles), and the toggle will measure 3' 6-1/2". Lay the pieces out on a large even surface, as shown, with the nicest surfaces facing down (that will be the front, you will secure the pieces on the back side). Using a carpenter's square, make sure all the corners are square. If you need to force anything into place, or fudge for a bad cut, that can be done as you glue and staple.
Apply wood glue to 4 corner blocks and two keystones, and lay them on the joints, making sure that the grain runs across the joint. Inset these pieces 3/4" from the outer edges of the flat, using a scrap of 1" lumber as a measuring tool (This is to allow another flat to be butted against it at a right angle, if necessary, without the quarter ply getting in the way). Use pneumatic staples or 3/4" screws in the pattern shown, to fasten them--Two screws on each side of a joint, making triangles (the shape with the most strength) where possible. Don't use more fasteners than shown, too many holes will weaken the wood.
**A good teaching tool, at this point, is to demonstrate how the joint will fold right over if the grain runs along the crack. Don't be afraid to do this to a student's work, it will have to be redone, anyway, if the corner blocks are on the wrong way. To remove the corner block, unscrew or pull staples,and scrape off with a chisel, if the glue is dry. If still wet, just pull up and replace. It's frustrating, but it happens all the time.
TV Flats- These are laid out in the same way, with the rails above and below the stiles, and the toggle(s) between the stiles. But, the lumber is laid on its side to give thickness. As such, the math is different. The stiles on an 8' tall flat are 7' 10-1/2", because the two rails are 3/4" thick, not 2-3/4". I only mention this because, it is easy to get confused on your cuts if you are using both kinds of flats in a show. If you cut dimensions for the wrong kind of flat, it will be too short or too tall.
To secure, glue and screw through the sides into the lumber. It is not necessary to have corner blocks etc., because TV flats are always skinned with wood, not muslin. The face serves as one giant corner block.
Soft Flats- Only standard flats can be used with a soft surface. Lay your frame on saw horses, cut your fabric a little larger than it needs to be. Paint the front surface only with a mixture, about 50/50, of glue and water (called "dope"). It should still be viscose enough to hold the fabric in place. Use your hands to judge the texture! Make sure to paint all the way to the edge, so the muslin doesn't curl up.
Gently, smooth the muslin out over the wet frame. After it is in place, use a hand stapler, sparingly, to secure the fabric near the inside of the frame only. Don't pull the fabric tight, just make it smooth. The glue and paint will size the cotton, so if it's too tight, now, it will be bad for you later.
After the frame is dry, use a mat knife to cut the excess fabric even with the outer edge of the frame. Now, paint the entire surface with the dope. This will size and harden the muslin, making a really nice smooth surface to paint on.
Hard Flats- You can use a hard surface on either
kind of flat- standard or TV. As before, lay your frame on
saw horses or a large table. Cut the Luan etc. to the exact dimensions
of the outside of the frame. Put glue on the front surface of
the frame. Lay the skin on and staple it into place, using an
electric or pneumatic stapler. Paint or paper, as desired.