So now you have the first two sides set up. Fix the rear wall log in place to prevent it from moving whilst you set up the remainder of the garage.
Using the same method as above, set up the opposite long wall log shown here in BLUE
Now add the 2 short returns that form the door opening.
Check the Diagonals for Square
Now, if the previous work has gone to plan, the next bit is purely for double checking !
Measure across the diagonals in both directions. If everything is in order and perfectly square, the distance across both diagonals will be exactly the same. Adjust as necessary !
By using the above methods, you should ensure construction of your timber garage starts in good shape, nice and square !