There is a little bit more to using a shower chair than you think. Especially if safety is your concern.
Here are the key steps:
Adjust the legs so that the seat is level Place the seat in the tub or shower Step over the tub edge holding onto a grab bar. Back up towards the shower seat until you feel it on the back of your legs Sit slowly onto the shower seat.
NOTE: if you cannot safely step over the edge of the tub, you may want to consider a product like a tub transfer bench instead. A transfer bench allows you to get in and out of the tub without having to step over the edge.