Practical guide and verification
State the model first
Basic projectile equations usually assume constant gravitational acceleration, no air resistance and a flat reference frame. Those assumptions are useful for textbook problems but can diverge from real motion when drag, wind or large distances matter.
Separate horizontal and vertical motion
Under the ideal model, horizontal velocity remains constant while vertical velocity changes because of gravity. Keeping those components separate helps avoid mixing angle, speed and displacement formulas.
Common mistake
Check whether the launch and landing heights are equal before using a symmetric range or flight-time shortcut. A formula derived for equal heights can give the wrong result when the target is above or below the launch point.