First off, they're one footed grinds so you obviously have to jump off of the non grinding foot.

For fastslides, jump up and forward so your momentum is keeping your forward. Lock your leg into place and lean far forward over your leg.

For backslides, have a deep groove in the plates so you can lean onto the brake bar. You want all your weight on this leg while slightly leaning forward to keep your speed.

On backslides, for me, I cross my back foot over my front, kinda, then jump on the rail. Really... it's all about keeping your weight sensored on that sweet spot.
For fastslides, well, I dunno, just...jump on your font foot I guess, that's a hard one for ME to describe.