Try to find a level part of your yard for the goal post. When it's complete, it will be about 12' high and dug 2' into the ground, and held secure by cement.
Once you've decided where you want your goal post, mark the midpoint of the area. Measure 5' and 1-1/2" on both sides of the stake, and mark those points with two additional stakes. These points will be the middle of the holes for your two 3" PVC support poles. You have to compensate for the size of the "T" connectors so the 10' crossbar will fit without being cut.
Use a two-man gas powered auger (available at most tool rental stores) to dig the holes. You'll need to dig 2' into the ground, so take a piece of duck tape and place it 2' on the auger bit. You should use three people for this job -- two people to operate the auger and one person to tell you when you hit the mark.
Note: These augers don't have an on/off switch so be sure the auger is in place before you start the engine.
While the auger will do most of the work, you should pull it out of the hole every few seconds to clear the excess dirt. After digging your first hole, move to your second mark and repeat the process.
Shovel any excess dirt out of the hole and tamp the earth at the bottom of the hole. Add some small or medium sized rocks to the bottom of the hole; this will provide a strong base for the cement.
Take 12" concrete cement forms and cut two 2' pieces. Place the form into the hole; this will keep the cement secure inside the hole. Once your hole is prepped, it's time to cut your PVC pipe and mix the cement.
Since we are going to use standard PVC connectors to attach the crossbars, you can build the uprights in three pieces. You'll start with two 10' PVC pipe as your base, sinking them 2' into the ground and exposing 8' as the first section of your upright. Later in the build, you will connect the shorter pieces of PVC above the crossbar line. The best way to cut the PVC is on a chop saw.
Note: Before mixing the cement, make sure to have everything you need close by -- six bags of quick drying concrete, a hose with good water pressure, a large, long trough to mix the cement, a hoe or rake for mixing and the PVC pipes.
Once ready, mix your concrete. You'll need about 2-1/2 bags for each hole. Add the water slowly, mixing as you go. You want the concrete to be the right consistency -- not too runny or not too thick to pour (Image 1).
Once the cement is fully mixed, start shoveling it into the hole. When the hole is filled, take the 3" PVC pipe and force it through the cement in the center of the hole. Have someone hold the pole in place for a couple of minutes and prepare a brace to support the pipe so the pipe doesn't move while the cement is drying (Image 2).
To build a brace, use two 8x2x4s. Stand them up against each other like a teepee (Image 1) and put a screw to hold them together. Then take duck tape and wrap it around the pole and 2x4s. Once it's in place, cut two pieces of 1x2, cutting a point on one side creating a stake. Have someone hold a level against the post while someone else adjusts the pole to make sure it's straight. Place the 1x2s at the ends of the 2x4s. Hammer the 1x2s into the ground at the appropriate point and secure them to the 2x4s (Image 2).
Once the brace is completed, start mixing the cement for the next hole and repeat steps 3 and 4.
While the brace is still in place and the cement is drying, it's time to work on the assembly for the rest of your uprights.
Note: We are building the uprights so the crossbar can be adjusted. We're starting with a crossbar at 8' and planning for a second position at 10'.
To complete the uprights, take a 10' x 3" PVC pipe. Cut it into four sections: two 2' pieces and two 3' pieces. You'll need four 3" PVC T-connectors for the adjustable crossbar. Since the "T" will serve as a support for the crossbar, you'll need to cut the middle part of your connectors, creating a cradle. Simply cut off the top half of the middle of the "T" connector with a handsaw so that the crossbar can rest on it easily.
Add the modified "T" to both the top and bottom of the two 2' pipes. Make sure the connectors are perfectly straight and lined up, then secure with PVC glue.
Note: PVC glue dries very quickly so make sure everything fits correctly before you apply the cement.
Attach the 3' piece to the top connector of each assembly. Remember, the two 3-way connectors are for the adjustable crossbar. One will be at 8' high and the other 10' high.
Once you've attached the 2' and 3' foot pieces, you're ready to attach the flags to what will be on the top of the uprights. You'll need two 3" PVC clean-out caps. This glues to the top of the assembly and gives you the ability to unscrew the cap at the top if you wish to change or remove the flags. Drill a hole in both caps and attach your flags. Then add your flag to the top of your assembly.
Take the two assemblies and carefully attach them to the bottom posts with PVC cement.
Attach the crossbar. You'll need two ladders and two people for this job. Place the 10' and 1-1/2" PVC crossbar into the T's and drill a 1/2" hole on both sides, 1-1/2" from the edge. Then take a 4" bolt and place it through the hole and secure it with a washer and nut. This will keep the crossbar in place until you're ready to change its position.