Actually a rectangular frame. This tutorial is for the frame only, without any decoration. I'm writing it after the fact, so the order and photos are less than ideal, but hopefully sufficient. It's really easy if you know how or can get a little help with a drill and circular saw.

Step 1: Gather Materials & Tools

Note, I'm listing the actual dimensions of the lumber. When you go to the lumber store, the standard dimensions listed will be different. (Standard dimensions) listed below in parenthesis.

3/4" x 3-5/8" (1" x 4") wood for the frame. This wood is usually sold in 8 ft pieces; I had to buy four pieces in order to have enough to make the sides, top and braces. I used a textured wood to make the arch look rustic.
1-1/2" x 7-1/4" (2" x 8") wood for the "feet" of the base that holds up the frame. Each "foot" is aprx. 30" long.
3/4" x 3-5/8" (1" x 4") scrap wood; four blocks cut to the width of the feet and tacked on to the ends of each to make the feet sit level (see notes in photo).
Four brackets or "angle ties" to hold the frame to the base; two for each of the "feet." I used these.
Four bolts aprx. 2-5/8" long to hold the brackets to the "feet" of the base. I think mine were 5/8" in diameter but just make sure they fit through the holes of your brackets.
Two bolts aprx. 1-5/8" long to hold each side of the frame between the brackets on the feet of the base.
Six nuts to fit the above bolts
Four smaller bolts aprx. 2-3/4" long to fasten the top of the frame to the sides. I used two bolts on each side.
Eight washers to fit these smaller bolts. You'll use two washers for each bolt.
Eight drywall screws, aprx.1-3/4" long to fasten the braces to the top and sides of the arch.
Black paint for the base, screw heads and nuts.

The tools I used included a circular saw, a drill, a carpenter's square and a paint brush.

Step 2: Cut Wood for Frame and "feet"

Make sure you know the dimensions of the space where you're going to put the arch before you start cutting, especially the height of the ceiling or tent, as the case may be. I used a circular saw to get a clean cut. The materials list has photos and dimensions of the different pieces of wood I refer to below.

Cut the top (if necessary) and sides of the frame. I left the top of the frame at 8 ft, and cut the sides down to about 7 ft.

Also cut:

- The two "feet" that comprise the base of the arch; each is aprx. 30" long.

- Four blocks cut the width of the "feet."

Step 3: Paint Wood (and Any Hardware)

Paint the top and sides of the feet of the base and the sides of the stabilizing blocks that get tacked on to the bottom. Also paint the heads of any screws and nuts that you don't want to be bright silver and couldn't find in black.

Step 4: Make the Feet

1. Pre-drill the bolt holes on each "foot" where you'll attach your brackets.

Choose your drill bit size to be slightly smaller than each of your bolts.

Before drilling, make sure you know where your want the holes to be. You want the sides of the frame to sit in the middle of the "feet," so you need to take into account the thickness of the wood that's going to sit between the brackets. In my case the wood for the frame was about 3/4" thick.

I set my brackets on the "feet," leaving a 3/4" space between them where the frame would go, and used the brackets as a stencil to draw the holes before I drilled.

2. Attach your brackets with the bolts, placing a nut on the other side of the wood for each.

3. Nail on the stabilizing blocks, one on each end of the "feet" so they will sit level on the ground.

Step 5: Pre-drill Holes on Sides of Frame and Fasten to "feet"

Pre-drill the holes on the bottom of each side of the frame where you'll bolt them between the brackets. Hopefully you measured well so that each side of the frame fits snugly between the brackets. Use a nut on the back of the bolts to tighten it up.

Step 6: Pre-drill Holes and Fasten the Sides to the Top of the Frame.

I laid all of my pieces flat on the ground and upside down to see how the frame would look. Then I made marks on the wood with a pencil before drilling anything.

I left about a 6" overhang on the sides and 2" on the top.

Use the smaller bolts (two on each side of the frame) to fasten the sides to the top, with with washers above and below the wood and a nut on the end of each bolt.