To frame a door in a shed building is a rather simple application that is almost identical to putting in doors elsewhere. You have to construct a double post on one side of the frame, where the door will hang, put in a header along with other upright two by fours. You also have to make allowances for the door jamb and door that will go into the opening.
First of all you have to decide where you want to place your shed door. At this time you will have to decide the exact dimensions of the door that will be installed. For simplicity we will decide on a door that is 76 cm (30 inches) wide and 2.13 m (84 inches) high, but shed doors sometimes need to be bigger than this.
Next we have to calculate the size of the opening. We must allow for a simple doorjamb (this is installed to protect the frame of the building from the weather) and then add an extra 1.3 cm (1/2 inch) to both the height and width. The extra space is needed to allow the door to open and close properly. The jamb will be made from 2 cm (3/4 inch) pine or fir, but there will be shims on one side, plus the top. So we add another half inch to the top and one side. That comes to an extra 4.5 cm (1 3/4 inch) for the height and 6.3 cm (2 1/2 inch) to be added to the width. So our opening will come to be 2.175 m (85 3/4 inch) high and 82.3 cm (32 1/2 inch) wide. Thicker wood means a bigger opening. If you use 2.5 cm (1 inch) timber for the jamb, then you have to add another 6 mm (1/4 inch) to the top and another 1.3 cm (1/2 inch) to the side.
Most likely you will be installing the door in a two by four stud framed wall with a bottom plate and top plate, so put a small square mark on the bottom plate, exactly where you want the door to be. Make a second mark across the 2 by 4 (that's the bottom plate) that is square and exactly 90 cm (35 1/2 inches) apart. Add extra 7.5 cm (3 inches) to the size of the cut to allow for both two by fours to go directly to the floor, where they then can be nailed into the end of the bottom floor plate with a few quick swipes of the hammer. Then just outside these marks you need to drive a couple of 9 cm (3 1/2 inch) nails to secure the bottom plate.
Remove any two by fours that are already standing within those 90 cm (35 1/2 inches).
Once this is done you need to cut the bottom plate with your circular saw, although an old hand saw might work in this case. Set the depth of the circular saw to match the thickness of the bottom plate and make sure your two cuts are perfectly square. At this point you are ready to start putting in two by fours to make the doorframe.
Decide which side you want the shed door to swing from. This will be a double post and you will install this first. Next take a measurement at least to the nearest 3 mm (1/8 inch) from the floor to the bottom of the top plate. Cut a two by four to this exact length and nail the bottom end into the bottom plate, which you have just cut. Since the two by four goes all the way to the floor, you can drive two nails through the upright stud right into the plate. Then put one toenail on each end, but leave the inside empty of nails.
Now take a 1.2 m (4 foot) level and make sure the upright two by four is dead level. Make a mark on each side of the two by four as it butts into the top plate; then toenail the top of the two by four to the top plate until it is tight and sits right inside the two marks. When you are finished recheck the upright with the level.
On the other side of the opening cut another two by four to length and nail the bottom in the same exact manner. However, when you go to nail the top use your tape measure to insure that this two by four is precisely parallel to the other upright.
Next you need to install a header at the exact height that you have calculated for the opening, 2.175 m (85 3/4 inches). With a small square make a perpendicular mark on each upright at exactly 2.175 m (85 3/4 inches) from the floor. Now cut two two by fours exactly to the size of the opening that exists on the floor. Theoretically it should be 85 cm (34 inches), but in reality the measurement will be a little bit shorter. Install these two pieces of wood between the two marks that you just made at the height of 2.175 m (85 3/4 inches). Make sure they are flush on the outside. They will not be flush on the inside unless you add a piece of 6 mm (1/4 inch) plywood between the two. Double check to make sure the opening is square and plumb. Measure and cut two pieces of two by for that will run from the top of your header to the top plate. Nail these in place and again make sure they are flush on the outside.
Add a second two by four to the side of the opening from which you will hang the door. This piece of wood will run from the top of the bottom plate to the underside of the top plate, so it will be approximately 3.8 cm (1 1/2 inch) shorter than the other two uprights. It will be easy to nail, but after you are done don't forget to check this post to make sure it is level.
Check to see if the floor of the shed is level. If it is not use the high side as your starting point.
Be careful with the electric circular saw. Always check to make sure the blade is tight and always set the depth of the blade to the minumun that is necessary.