Wash your jar in hot, soapy water and remove any labels. Dry the jar completely. Cover your work surface with newspapers and have whatever cleaning solution you'll need for your brushes already set up. If you're going to do an outline, test the outline paste before you start, to learn how much pressure you need to apply to get it to come out evenly from the tube. If you make a mistake with either the outline paste or the paints, it's easy to correct by wiping it off with a paper towel before it dries.
Paint your outline first. If you're working from an existing pattern, you can tape the paper pattern on the inside of the jar and follow it from the outside. Or, you can work freehand. Let the outline dry according to directions, then paint with transparent glass paints. Hold the jar up to the light periodically to see how it looks. It'll look different with the light coming through it.
Allow your design to dry thoroughly (this usually takes about two days). To turn your glass jar into a lantern, put a layer of sand on the bottom and add a tea light.