build the trusses on the ground first
Lay out the length of the side, .... 8ft, find the midpoint, ...4 ft, set a vertical to approximate the height and then locate your rafters, the length needed to for your canopy to fit. You adjust the vertical to suit the peak of the canopy and the rafters will adjust themselves automatically if there is just one nail on the vertical to pivot on. Let them run "long" and then cut them after it all fits.
Then attach the collar tie to hold it together. Set the first truss on the top plate then allow the second truss to slip under the first one with a gap for the thickness ...1 1/2" at the top. Vou don't need a fancy block if your are just using 4 rafters. Once you get the 4 midpoints and want to do the corners you can just cut them to fit ....no math needed for the lengths.
I would not attempt to build this "in the air" from a ladder.... except for the corner rafters.
The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)
Last edited by woodnthings; 06-05-2013 at 08:54 AM.