There are a thousand ways to skin a cat, and easily ten different ways to teach a sit
It sounds like you are looking to teach a "tuck sit". There are different types of props, typically a type of object that is usable as a perch to teach the puppy to remain stationary with the front feet on the perch. I personally like to use the small black rubber feed pans that you can find in farm supply stores -
You can use a perch of this object to teach many rear end awareness exercises as well as positions. The first step to teaching anything based off of perch work, is to teach the act of perching itself, before asking for any additional exercises. I'll link a video below showing a puppy demonstrating the concept. You'll see me toss the food away from her and ask her to come back and stand with the front two feet on the perch. In the beginning stages, I will usually lure with food onto the perch, and I'll repeat that exercise until I see that they have a definite understanding in that they begin to offer standing on it without influence by luring.
I would begin teaching rear end awareness exercises at the point that they know that the game it about keeping the front feet on that perch. I encourage the puppy to spin in circles, either by luring the behavior or shaping it, and waiting for them to offer just little pieces of movement at a time. Rear end awareness exercises in general will aid the clarity of your positions in the long run. I'll link a video of another puppy at the next point in progression, spinning on the perch.
As you make progress working on those rear end awareness exercises, I would begin to introduce luring her into the 'tuck sit'. First, the puppy needs to be familiar with luring exercises, and committed to following the food. Then, I introduce it by starting the dog from a down position. I will be looking to pull them up and forward from the down position. I demonstrate it in this video, around the 1:45 mark.
I hope this makes some sense to you! Let me know if I need to do a better job explaining.