how do i measure and cut to install a door in my block concrete garage wall. i have a 36'' metal door.

Question

also have 2x8's to frame it.

Answer 1

Assuming your door is pre-hung, measure the jamb width (not the molding attached to the face of the jamb). Add 3 1/4 inches for your 2x8 lumber, ( you did buy treated didn't you?). Add another 1/2 inch for room to plumb the jamb in the opening. (I'm afraid you have to go back to the store to buy shims...get some "great-stuff" for doors and windows while you're there.)
Measure height of door jamb. Add 1/4 inch for leveling. Mark wall using level. Cut blocks with concrete saw you rented while you were out buying shims and great-stuff. Pray wall doesn't tumble down on your head, ( You did remember to consult a qualified engineer. It's much better if you pay them to take the blame when things go wrong.) Fill voids in wall with mortar or cement. Tapcon treated 2x8 to opening using a generous amount of liquid nails I forgot to tell you to buy. (Back to the store again...you actually had tapcons and a hammer drill?).

Finally...the door. Get out your six foot level and your three foot level. Shim bottom of door on hinged side about 1/4 inch. Plumb hinged side up shimming as needed. level across top. Plumb down other side. Use shims behind every screw. (You did buy screws). Fill voids with great-stuff. Caulk. Paint. Lightly oil hinges. Enjoy :)