But I'm not here right now to tell you what to do with them (maybe later); I'm here to tell you how to clean them inside and out, using safety precautions.
The need to clean depends largely on what you plan to do with the gourd. You may decide to only work on the outside without a need to open the gourd, in which case you get to avoid the extra work involved. If you plan to make a bowl, for example, then clearly you'll be working on both inside and outside, as well as cutting and/or carving.
The outsides of dried gourds are fairly simple to clean, if sometimes labor intensive. The extent to which you clean them depends upon how you want to use them, and what you might want to add to the surface (paints, dyes, etc.). If you want a naturey-looking gourd with its natural waxy coating you don't have to do much beyond washing off dirt and dark mold with a hot water and bleach solution (maybe 10 parts water to 1 part bleach), which retards future mold growth. However, if you want to dye a gourd, leaving the waxy coating on it might prevent dye from soaking into the surface the way you want it to, which means you'll have to scrub harder to remove nature's irritating addition to your innocent craft project.
The insides of gourds are a different story, and a potentially dangerous one. You need to be careful not to inhale the contents. Not all hardshell gourds are terrible inside, but they are all dusty, and you can't guess which ones will merely make you sneeze and which ones might contain mold/fungus, bacteria, and other pathogens that could cause serious allergic reactions, including anaphylactic shock. I've personally been flattened with a gourd dust-related illness that gourd crafters refer to as "gourd flu", which was basically illness that mimicked semi-severe flu symptoms (fever, aching joints, muscle soreness, mucus, fatigue) for about two to three days. If there's a medical term for this, I don't know what it is, but in hindsight... I probably should have seen a doctor. And you should, too, if it ever happens to you.
So... to prepare, make sure you use a dust mask. They're easily found at hardware stores all over. Choose a good one designed for working with wood/sawdust; if possible, look into one that also protects against molds/fungus. Second, use protective goggles. Remember, these precautions aren't just for gourds; any time you do craftwork involving dust, solvents, tools and so on.
What can you expect to find when you open a gourd (via Xacto knives/saws, mini electric jigsaws, or Dremels... be careful not to cut yourself)? Dust, seeds, a styrofoam-like substance, and hardened pulp, maybe even a few tiny beetles. You probably don't want to keep that junk in there, right? So you have to clean it out, and often this is easy, but occasionally this can get ugly.
You should prepare yourself for cleaning the outside with:
~ A sink or tub, the size depending on the size(s) of gourds you're cleaning.
~ Copper scrubbing pads. 100% copper means they won't rust should you want to use them again later.
~ Old towel(s)
~ Something to scrape with, perhaps a knife edge
~ Rubber dish gloves
You should prepare yourself for cleaning the inside with:
~ Gardening gloves, or thicker gloves
~ A melon baller
~ A taxidermy scraper
~ A sanding sponge
~ A shop vac
~ Dust masks, the best ones being for the finest particulate matter
~ Eye protection, especially if you plan on using power tools
~ Electric drill with wire brush bit for spots that are hard to reach
Please don't ignore the safety precautions, especially regarding sawdust. If you use a high-powered cutting tool (Dremel, etc), you'll be generating a lot... a LOT... of sawdust from the gourd, and it is very, very fine. The faster the tool, the more dust you'll create. Many gourd artists opt for mini-jigsaws with variable speeds, to control the cutting and minimize dust. In addition to dust masks and goggles, you should work in a well-ventilated area.