Don't waste time trying to learn from sped-up videos or thinking you have to master everything at once - cumbia is a dance that requires you to *feel* the rhythm, not just memorize steps. What matters is starting slow and letting your body get used to the movement.
The basic step is pretty simple: your feet follow a three-beat pattern while your hips move naturally. Stand facing your grandmother and do this: with your left foot, step forward (beat 1), then bring your right foot back without putting your full weight on it (beat 2), and on beat 3 you put your weight back on your left foot without moving from where you are. Repeat the same thing but starting with your right foot. Your hips follow this movement in a swaying way - it's not a sharp twist, but a smooth sway that comes almost on its own when you keep your shoulders relaxed. Cumbia has that special touch where part of the movement is really subtle, so don't expect exaggerated movements.
Once you've got the basic step down, you can try the turn: basically you spin around on yourself while keeping that same foot rhythm. The best thing is to dance with your grandmother so she can correct you or just let yourself be led by her. Cumbia is a social dance, so it's not about being perfect - it's about having fun. Trust me, after a few minutes of practice, it'll come naturally to you.