Identify the stem system
First determine whether your bicycle uses a threadless or quill-style stem. They attach differently and are not direct substitutes. Check the bicycle documentation or ask a mechanic if the system is unclear.
Measure both clamp interfaces
The stem must match the steering tube interface and the handlebar's clamping area. Measure the actual clamping sections or use the dimensions printed on the existing parts.
Record length and angle
Stem length is normally considered from the steering-axis center to the handlebar-clamp center. Angle and orientation change bar height and reach. Even a compatible stem can noticeably change handling.
Treat installation as safety-critical
Use the manufacturer's fastener sequence and torque values. Confirm that all minimum insertion, spacer, and clamping requirements are satisfied before riding.