mermoose, your message to me mentioned my avatar. thank you for your kind words. i have been using colored slips since 1972. it is a very wonderful substance for decorating pottery. it is one of potters' favorite methods and has been used for centuries all over the world.
slip is simply very wet clay. the kind of clay you are using to make the pot will make slip. just keep the trimmings of your pots dry and save enough to fill a container. add some to water. let that container sit overnight to thicken. you can add color to the wet clay and cover the pot or part of the pot with this wet clay and let it dry. if you brush this slip onto your wet pot, you have simply thickened the clay that will become the pot by a layer of additional clay that contains color. it becomes part of the pot, not something additional that might need an adjustment in chemicals. keep it simple.
this works so easily that there is probably no reason to add other ingredients. if the clay you use normally is very grainy, you might choose to screen out the large grains before applying the slip. i used to use this kind of clay but do not anymore.
since the slip is simply wet clay, the time to use it is immediately after throwing when you can see the shine of water just leaving the pot you have made. it can be added later by dampening the pot so it is in a condition to receive the wet clay but it is much simpler to do it when the pot is in the correct condition. putting pots inside plastic bags and keeping them damp works to preserve the correct condition until you are ready to carve later.
there are many tools and ways to carve through the new layer of color into the underlying clay, the part you formed into the pot. when you finish your design and let the pot dry totally, you will see that there is a difference between the clay of the pot and the colored covering you put on the outside. a transparent or translucent glaze will allow this difference to be seen and admired.
this is a very simplified version of how to use slip, colored or not. there are infinite possibilities for the use of slip. one of the very best books for a newcomer to this style of work is a very old one written by Charles Counts. it is called Pottery Workshop. it is not perfect, some glaze recipes have errors in them but it contains basic knowledge that is of value to beginners.
if you will S T U D Y this simple book with its progression from how to throw a simple small cylinder to the final decoration of all the styles of pots shown, you will have enough knowledge to take you in any direction you want to go.
you tube is a valuable tool. but you need a foundation of knowledge to use any tool well. please start at the beginning, you will find it a better way to proceed.