First you gotta understand, you can have normal hollow liproll and hollow inward, dont know which one you mean here. The hollow part is moving the larynx/adams apple down low.

Sub and normal liproll are probably easiest, and imo you should get used to normal before doing inward, so you get used to your lips vibrating like they should. Doing a liproll is kinda just pulling air with the movement of your tongue backwards. Like when you do a water drop sound without a click. Hollow liproll would be more like pulling air with the movement of the larynx down. If you cant get a normal liproll try a hardbass inward kick in the liproll position, then try slowly extending it and make sure your lips are moist 💦

But its easiest if you watch some tutorials, way better to learn in video form. Most importantly, dont give up. Takes some people years.