ALWAYS wait for the car to fully stop before engaging reverse, preferably still in gear whilst your braking and clutch disengaged.
If you shifted into neutral whilst stopping the car, wait 2-5sec after disengaging the clutch before selecting reverse.
The reverse gear has no synchro and involves sliding a separate straight cut gear cog between the input shaft and main shaft to "reverse" the rotation.
This means that both the input & main shafts & reverse cog MUST be speed matched before they can slot together without crunching.
Since the reverse cog is totally isolated from the spinning shafts when it's not engaged, it will be stationary upon selection.
So this dictates that both input & main shaft MUST be stationary before the reverse cog can be inserted.
The main shaft always follow the differential/wheel speed so if the wheels are still, the main shaft is also still.
Whenever you disengage the clutch whilst above idle rpm AND immediately disconnect the input shaft from the main shaft by selecting Neutral before braking, the input shaft and clutch disc are still spinning quickly with some inertia. Reason for waiting 2-5s here is to allow the input shaft to decelerate to a full stop before the reverse cog can be inserted.
When you disengage the clutch & brake BUT leave it in gear, the input shaft will follow the main shaft. So when the wheels stop, the main & input shaft also stop so you can immediately engage reverse gear.