I can't get my lips to trill when I fully relax my jaws(it was the instruction to follow before doing lip trills). I can only trill them when I put some pressure on my cheeks or put two fingers at the side of the cheeks. But how did Jonathan Estebrooks did it with the jaw relaxed? If I put some pressure at my cheeks(without using fingers), will that cause tension?

No it does not. Applying the fingers in howls of your cheeks should facilitate the lip trill. Make it easier to produce the buh buh buh. 

What Jim said! Totally fine to use your fingers for support. 

Alright!