1-take a small amount of the gouache/watercolour, and squeeze it into the cup.
2- add in a few drops of gall. Six or so drops is a good starting point, but some colours need more gall than others.
Once the colours are mixed, you can begin testing them.
Firstly, "skim "the size. To do this, drag a piece of paper or a slat of wood over the size. This is done to remove the "skin" on the size, ( much like a soup), which will stop the colours from spreading.
Then drop on your first colour. It should spread like this; to a circle about 2-3 inches across
2nd colour onThere is no fixed amount of gall to be used. Some colours require more than others. However, If you are using HUGE amonts of gall, to no effect, something is amiss. Either
1- Your colours are too thick - Try watering them down
2- The gall is weak. This is a common problem with gall meant for watercolour. - switch to a diffent brand, or one from a marbling supplier.
If, however, the colours do not spread much, and you have found that the gall is not the culprit, then there is something wrong with the size.
3- The size is too thick - Add some water to thin it down
4- Size too cold- add some hot water
5- Did you skim the size?
Sometimes, you may find that the colours look grainy on the size. Not to worry, add some drops of alcohol.