The standard skid plate is quite thin and easily bent, the more out of shape it is the more difficult to refit.
I find they go back on without too much trouble if their in good shape
Fit the front bolts first just loosely and offer the rear up to the frame, watch out for the engine rubber supports moving about, then fit rear bolts.
One badly out of shape skid plate we had we loosely fitted the front bolts then put a car trolley jack under the skid plate and took the weight of the bike on that then fitted rear bolts.
Be careful not to OVERTIGHTEN engine casing bolts when refitting the engine casing, use torque settings supplied in the manual, i've heard of a couple cracking when overtightened.