How to make layouts mobile responsive

[carisma_sensei]

Hi! It's quite obvious that, nowadays, everyone uses their mobile phones to browse websites most of the time, at least when they are out. I've been told my layouts are not mobile responsive, which is true and I knew, but I've been wanting to try my hand at this for a while and so I'm asking, how can I go around doing that? What do I have to know, which tutorials or documentation would you suggest me reading? The styles I use mostly are Transmogrified, Tabula Rasa and Practicality.

Thanks!