There are two main reasons for crunching into reverse.

1. You have not allowed the rotating gears to come to rest after depressing the clutch pedal. If the box was in neutral, the input shaft & cluster gear are rotating. Depress clutch fully & wait a second or two to allow gears to stop..... Select reverse.

2. The clutch does not fully release & the resultant drag on the clutch disc prevents the gears from coming to a stop. This can be caused by
A. Lazy leg syndrome [push harder]
B. Clutch badly adjusted. [adjust it]
C. Oil on clutch from engine or gearbox [Pull box out & rectify cause, then replace disc, or whole clutch]
D. Unservicable or faulty clutch. [Replace it]

Reverse is the only gear without synchro, & two gears actually slide together, unkike the foward gears which use a diferent system alltogether. One of these gears will be stationary if the car is stationary, so if you try to engage a rotating gear with a stationary one,... well you know.
Bring the rotating gears to a stop & they can not crunch. If you like, you can just "lean" on one of the foward gear positions to use its synchro as a brake for the rotating parts, then select reverse.
Try not to select reverse while traveling foward, it's bad for the nerves, the box, & your wallet.