The simplest way is to just pour them from the pot or squeeze them from the bottle.
To accurately measure out ratios, you have a number of relatively easy options, I think. One is to take a small container that already has gradations printed on it, so you can just put (say) 3 ml of one colour and add 1 ml of another. Alternatively, you can mark those yourself on the outside of the cup with a thin permanent marker, provided the cup you use has straight (or nearly straight) sides: for a 3:1 ratio, make a mark 3 mm up from the bottom and another 1 mm above that, so 4 mm from the bottom — or 6 mm and 8 mm, of course.
What you can also do is use a pipette to draw (say) 3 ml of water and put that into the cup you use, and with the cup on a level surface make a mark on the outside at the level of the water. Then add 1 ml more water and make another mark. Throw out the water, rinse the cup, and put paint in to those same lines. Unlike measuring along the outside with a ruler to make marks, this will work with any shape of container.