That's a super broad question, could you be more specific?

The way I design a responsive page is to first create the desktop version, and add code to transform it to the tablet version, then the mobile version.
I then go through my code and reverse the logic so that the mobile version is the base code, with additional code to transform it to the tablet version, and even more code to make it into the desktop version.

I find this method to be the easiest as desktop tends to be harder to code due to more elements on the screen, so it's easiest to start it from scratch. I then make it 'mobile first' because it loads quicker on tablets, which often have slower internet speeds due to 2G / 3G networks vs the wifi of a desktop.