Paint either horizontally or vertically, but try not to do both on one project, as there may be a slight texturing of the surface that may have a different appearance when you look from various angles. Always clean the paint sprayer gun thoroughly after use. For dried-on oil-based paints, you may have to use acetone or lacquer thinner. Read the instruction or operator's manual for your sprayer. You should be familiar with the capacity, viscosity, and type of paint your sprayer will apply.
The controls on the sprayer used in the photos are fairly typical for this type paint spray gun. The upper control valve regulates air volume; the lower one meters the paint flow. The front of the nozzle is held in place with a threaded ring, and the pattern can be changed from vertical to horizontal by turning it. Mix enough paint for the complete job, if possible, since any future mixes may differ slightly.
Use hot water to thin water based paints (about 50 degrees C or 122 F). You may only need to thin acrylic paints by 5% when using hot water. Use a catalytic reducer for automotive finishes. It is specially formulated to speed the drying time and prevent runs without affecting the finish or color or the paint. It wouldn't hurt to use an air line filter or a dryer to remove moisture and dirt from the compressed air stream.