I want to get them but every time I try my kart goes out of control and I usually don’t get it and if I do half the time it saves no time

Do you just mean MTs in general or specifically SSMTs (straight stretch MTs)?

Spasticity ssmts I don’t feel on a straight I can get an yellow e to a orange e boost and I usually go into the grass or just outside the plains Any tips

I'd recommend turning a little to the opposite direction before hopping into the slide. Like if you were on the start of Luigi Raceway and wanting to do left-facing MTs, before jumping into the slide, angle your kart a bit to the right so that your angle when you complete the MT isn't quite as bad. Over time you'll improve and won't have to angle the kart as much before jumping into the slide.