If solid black color is what you want then paint will be the easiest way to go. This is a fairly simple project. The steps are prep, prime and then paint.
One thing to consider is will you take the buck beds apart or paint them assembled. Either way doesn't really matter, it's a matter of convenience and if you want to do this inside your house.
The amount of prep depends on the condition of the different pieces and your own expectations.
Start with a good washing to remove greasy finger prints and the like. Any mild household cleaner should work. Allow to dry then do some sanding with 150 or 180 grit sandpaper or sanding sponges. The sanding will help the primer stick and produce a smooth surface.
Any deep gouges or imperfections can be filled with spackling paste or Bondo, Bondo is the best.
Any good wood primer will work. Something that dries fast will be a major plus. You can use either an oil base or acrylic. The oil base is very sandable but sticks acrylic is easy to work with.
Either way you can brush, roll with a small tight paint roller or spray the primer. The method you use will be the same you use for the finish paint.
Plan one two thin coats for the best look.
Use a good paint that is readily available in your area. You have the same choices as with the primer, oil or acrylic. For simplicity and fast drying I would use a good interior acrylic. The sheen is another consideration. It's up to you, but I would use a satin or shinier paint.
The paint will be applied in the same manner as the primer. Also multiple coats is better than one or two thick coats.