Total beginner tips on posture strumming etc

Im 4 days in and have been using Lauren Bates videos. I know basically nothing about music so im starting from scratch.

My fingers seem to slide of the strings and I sometimes end up strumming the wrong string.

Heres a video of some finger excercises ive been doing, please help a girl out, any tips are appreciated.

Also, how long do you recommend practise to be per day? Should i stop when finfers start cramping or does powering through make me a stronger player