Attach the computer’s mount to the handlebars per the product’s instructions. The method of attachment varies widely by model and make. The mount will be either zip-tied or screwed shut around the handlebar, depending on the brand. Then slide the computer into the mount after installing its battery.
Attach the speed magnet to the rear spokes if you plan on using the bike on a wind trainer when the front wheel will be immobile. Otherwise, attach it to the front wheel’s spokes. Match the grooves on the magnet to the spokes. The magnet may come with an adhesive to stick it on carbon spokes.
Insert the corresponding battery into the speed sensor and attach it (usually with zip ties) to the fork so as to be flush with the magnet. Rotate the wheel back and forth, fanning the sensor with the magnet to see if you can generate a non-zero speed reading on the computer. Adjust the position until you get a reliable reading.
Insert the battery into the cadence sensor, if there is one. Zip-tie it to the left side (not the drive side) of the bike—specifically on the chainstay. This is the near-horizontal part of your frame between the bottom bracket the rear dropout that holds the rear wheel’s axle. Make sure its sensing side faces outward. Mount the cadence magnet on the left crank, so the magnet faces the censor. You may have to adjust the positions of the magnet and sensor until they are close enough to get a reading. Lift the rear wheel off the ground so you can fan the left crank back and forth in front of the sensor, to test if there is a cadence reading on the computer.