You can apply a thin coat of either with a paper towel or a brush. Just like acrylic paint, you can thin these mediums with water but since they come already quite fluid I rarely need to. I tend to use these mediums more thinly when I am in the process of painting as opposed to the end when I am putting a final coat of gloss or matte medium over my art as an isolation coat.

A very thin coat will work fine. A little goes a long way, and you can always add more. If you are applying a thin coat, just wipe it on with a paper towel much like you would put suntan lotion on your body. Just apply a thin coat everywhere as evenly as possible.

When adding gloss or matte medium as the final layer to your piece, however, it is important that you make sure that the gloss is evenly applied over the entire piece, and that you are doing it on a level surface. You want to make sure that when the gloss medium dries it is not pooling in one area. If you pour too much on you can always wipe it off with a paper towel while it's still wet.