To make it harder find a short, low flat box or fat tube.
First times you try this you definitely rather want to lean a bit to much forwards than backwards.
When you lean to much forwards you can avoid crashing by just putting down the front foot on the box or rail.
Approach the box or rail you are trying this on with a gentle wind up and set of the rotation so you spin fast enough to rotate the 90° on.
Rotate 90° on to the box and also make sure you land with the ski completely flat against the box.
Try to to not rotate on the box so you are on the nice 90° angle even at the end of the box. Having you hands a bit wide helps slowing down the rotation.
If you have to, use a bit of counterrotation to adjust your rotation so you land nicely backwards. If Gary here did not counterrotate he would have landed crooked which isn´t smooth.
I often have my hands high and wide for good balance and it also helps you to slow down the rotation while you are sliding then when I jump of I pull in the arms to speed up the rotation again so I land switch in a nice way.
Having the arms high and wide can look pretty ugly but it is so practical since it help me to be able to to backslide 270s out with ease.
To backslide a whole box or rail you must in general lean back more than you think.