basically you have to check your position at the start

make sure the body is centered & forward

with the weight distributed on both skis

slightly a bit more on the downhill ski

move forward to get a little bit of momentum on your edges

and then flatten your skis and let them slide sideways nicely

then slide again and grip again

make sure you're using knees and hips at the same time

you have to make sure that the body is well centered on the skis to be able to do a proper sid slipping

don't start with the knees close togheter, otherwise you'll be catching the uphill ski edge

and to stop you just go back onto the edges with this movement of knees and hips into the hill.

I hope this will help, see you next time!