If you want the plane to go through the air in a level (as observed as if you were sitting in the cockpit) you just prop or jig up the fuselage to that attitude and then measure then angle of the wing(s), stabiliser, engine thrust line etc. Normally (if a monoplane) you will want a bit of positive angle on the wing, the stabiliser somewhat negative with respect to the wing and some down thrust and right thrust on the engine/motor. How much angle? Usually the plans indicate a starting point but if just guessing with a symmetrical airfoil on the wing you will want about a 2 to 4 degree positive on the wing with the stab about 3 degrees less with about 3 degrees down thrust and 2 to 3 degrees right thrust. Now most kits or ARF's will have the wing angle pretty well locking in by the way the fuse is built so, in that case, just use that as a reference and set the stab to a negative 3 degrees with respect to the wing.