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.
Step 1: Assess your gourd gunk.
Gourds, which are varieties of cucurbits (cucumber family) grow with a waxy green skin. The shell itself is hardened when dried out in the field, which can take weeks or months. The waxy skin shrivels, dries and turns the dark brown/grey you see in the photos of the intact gourds. The lighter tan you see is the hard shell itself exposed when the waxy skin is removed.
Now see the photos of the opened gourds. You'll notice some are fairly empty, and some are full nearly to the top with dried vegetable matter and seeds. Some are crusty and dusty, and some look pretty smooth. Some have a curled, dried skin inside that is easy to pull out, and others have a white, styrofoam-like lining which isn't so easy to clean out.
Step 2: Cleaning the outside.
What I do have is a photo of a gourd that didn't need much soaking and did well with just some hot water and a copper scrubbie. You can see I wasn't wearing gloves, which was a poor idea, because my wet fingernails were shredded. Yuck. Wear gloves.
This gourd is mottled from fungus, and as a result wasn't very pretty once clean, as the thick fungus darkens parts of the shell while the sun bleaches out parts with thinner or no fungus. If your gourd is too mottled for your liking you can set it out in the sun for a few days, which will fade it slightly, or you can try wrapping it in a bleach soaked rag for a few hours (then rinsing and drying). Neither option is guaranteed to lighten the shell, but occasionally it works.
If you have a gourd with waxy areas on the shell you'll have to soak longer, scrub harder, and possibly resort to scraping the wax off with the edge of a knife. I won't lie, this can be a huge pain in the patootie. But if you want dyes or inks to evenly saturate your gourd shell you will have to remove the waxy skin.
You can scrape the outer layer of skin off green (as in not yet dried) gourds, which effectively removes the waxy substance before the gourd dries, saving one the labor of scraping off the hardened stuff later. However, this can result in a collapsed gourd due to uneven drying based on the skin's removal (even during the drying process, the skin does offer protection against the shell drying out too quickly). You decide whether the risk is worth it. My luck with this method has been 50/50 at best.
Step 3: Cleaning the inside.
Now, put on your gloves if you don't want to get scraped up. If you do want to get scraped up, don't bother.
Dump out what you can.
Now grab your taxidermy scraper and your melon baller, which should be sufficient in scraping out all of the stuck-on matter from a typical gourd, and start scraping. And scraping. Scrape out every last bit of fiber, dust and seeds, dumping the mess into the garbage as you go, or vacuuming it out with a shop vac (recommended). If there are areas in your gourd that are tricky to reach, try using a power drill with a wire brush bit around those bends. You can also try bending long-handled spoons to reach odd areas, but it's not very effective in most cases. At this point you may end up with a very clean vessel, and if that's the case you're lucky.
A lot of gourds, though, have stuff inside that resembles thick, squishy styrofoam, and it's very difficult to scrape out. In this case you may want to resort to soaking. Set your gourd in a sink or tub, open side up, and fill it to the top with very hot water. Let it soak for a half hour or so, and when the water is cool enough you can stick your hand in there and start peeling that foamy stuff out. Whatever is left after peeling you can now scrape out with your melon baller or taxidermy scraper. You'll need to let the inside dry out before moving to the next step: sanding.
If you want the inside walls to be smooth you'll need to sand them. You may have a little piece of "stem" (umbilical cord? LOL) in the bottom of the gourd. You may ant to remove it with pliers, as not to scrape your hand on it while sanding. Use a sanding sponge, which easily adapts to the curves, to rub the walls smooth. You can then wipe or vacuum it out, and your gourd is ready for whatever project you have in mind.
Have fun. I'd love to see your finished projects if you'd link to them in the comments :)