Actually, SimpleFit doesn’t limit you to any single grip, it mostly depends on which one you feel more comfortable with; if you’re proficient in both, try alternating between them. Here, let me quote the details page on it:
Grasping a bar, pull your body up until your chin clears the bar, then finish by lowering your body until your arms and shoulders are fully extended. Use whatever grip feels right and it is often good to alternate grips.
You can do whatever kind of pull-ups you can do. Ideally, you should be working towards doing regular full range-of-motion pull-ups (palms facing away) or a combination of pull-ups and chin-ups (palms toward you). But, many people start before they can do a single pull-up. They usually start with either body rows (essentially a pull-up with your feet still on the ground–you can see lots of examples on youtube), or with negatives (jumping up to the top of the pull-up position and then letting yourself down as slowly as possible).
Some people work through the levels using the substitutions, while others stay on level one with the substitutions until they are able to do full pull-ups or chin-ups.
So, welcome to the Simplefit community! Now you can start using this board, as Bri3626 suggests to log your progress.