Summarize:

Joint compound—sometimes called sheetrock "mud"—is very fine dust mixed with water. There are two typical choices for skim-coat materials:  Pre-mixed joint compound comes ready to apply to the surface. After application, it gradually dries out. As such, you can add more water to the mixture to extend your working time. If you have never applied a skim coat before, you may find it easiest to use a ready-to-use pre-mixed product.  "Quick set" is made from the same dusty base as pre-mixed joint compound, but you must mix it with water before use. Setting compounds are like concrete: they don't dry out. Instead, they undergo a chemical reaction which makes them "set." Spackling is often mistakenly used as a skim-coat. However, spackling is harder to spread, more difficult to sand, and is best used on wood trim to fill large imperfections. These could include:  A ladder or scaffolding to reach high places without fatigue. This is especially important if you are applying a skim coat to a high wall or ceiling. A large five gallon bucket for mixing the skim coat. A metal mixing rod that attaches to a drill. This will make it easier to mix large amounts of compound. A mud pan. The skimmer plate. This holds the prepared compound. You will hold the skimmer plate in one hand—or place it somewhere within easy reach—while you apply the skim coat.  The compound applicator of your choice. You can use a paint roller or a flat, trowel-like "compound applicator". The applicator should be 6" wider than the area being smoothed.  Use a 12" applicator for leveling. A setting compound ("quick set") comes in bags, and you must mix it with water before using it. The bags have a time limit printed on them—often 20, 45, or 90 minutes—which indicates the working time under average conditions. Heat shortens working time and cold lengthens it. Mix your compound in small batches: if you mix too much at one time, it will start to dry out in the bucket before it can be applied.  The benefit of a setting compound is that it can be sanded or re-coated as soon as it sets.  This also means that you have know exactly where you will apply it and be ready, because as it stiffens it cannot be re-wetted. Setting compounds are much more durable than "mud" and will not come apart when wet. They are ideal for walls and ceilings in places that have been exposed to moisture, such as bathrooms and kitchens. A glop of setting compound will set even if dropped in water. Stir a bucket of ready-made joint compound with a paddle attached to an electric drill. Mix until the compound is completely smooth, adding water as needed. The resulting mixture should have the texture of custard. You can tint many joint compounds by adding tint while you are mixing. Find tint products in a hardware store. You can also add sand or other rough materials if you want a certain texture embedded in your coat. Start slow with the drill until the liquid is mixed, then gradually increase the speed. You can slowly add more liquid if you need to thin the compound. Run an image or video search for "mixed joint compound" to see what your compound should look like when "ready."  Mixing compound is much like mixing cake batter. Remember not to pull the drill mixer out of the compound while the drill is running, or you may have flecks of mud flying everywhere. It is critical to make sure there are no lumps in your ready-to-apply compound. If you do come across a lump of dry compound during application, you can probably smash it into the surrounding wet compound. If the lump is too big to crush, remove it with a small putty knife. The five gallon bucket needs to be cleaned each time you use it, or else little pieces of dry compound will carry over to your new batch. Your helper can transfer the prepared compound from the bucket into a smaller container. From this container, use either your applicator or a smaller trowel to move the compound to the mud pan. Then, your helper can start cleaning the bucket and preparing the next batch of compound.
Find a good joint compound/mud. Do not use spackling. Gather your tools. Mix "quick set" according to package directions. Stir a pre-mixed joint compound to loosen it for application. Add any tint that you want to use. When mixing, start by adding the least amount of water called for. Ask someone to help out.