A stellium is a cluster of three or more planets in the same sign and/or the same house.
To interpret a stellium, the easy part is to say: "this sign and/or this house are very emphasised in your chart" and taking it from there, talk about all the themes, concerns, personality traits associated with this sign and house. You'll get good results already. A stellium in Capricorn means that the person is an achiever, and will do what it takes to reach their goals.... unless, like with a single planet, the stellium receives powerful conflicting aspects, in which case the interpretation will have to mention the need to adjust consciously the energies and attitudes in order to make the best out of it.
The more difficult bit.... is actually no more difficult than interpreting a whole chart with ten planets placed in various signs and houses and making aspects to one another. You can analyse each planet one after the other, starting with the more personal ones. If the Moon is part of the stellium, start with what the Moon means in this sign and this house. Then, as in most charts, the Moon receives a few aspects, in the case of a stellium it's likely to be a conjunction or two... so take the second most personal planet, or the closest to the Moon according to your taste, and interpret what Moon conjunct this planet means. For instance, if Mercury is there, interpret Moon conjunct Mercury. The interpretation of Moon conjunct Mercury is very similar to Moon in Gemini or Moon in Virgo (check which one of these two signs is the more powerful in the chart). -- The other way round, Mercury conjunct Moon is similar to Mercury in Cancer.. .
At that stage, you start feeling you are about to get an headache, it's time to remember that astrology is a difficult art and that in order to use your intuition, you need to give a break to your intellectual mind; take a few deep breaths; let you brain take the time it needs for its own chemical processes. Stretch your body, look by the window. Put yourself in a relaxed dream mode. Imagine what kind of character a Moon conjunct Mercury in this particular sign and house could be.
At that stage, there are already too many bits of information for you to be able to process them all consciously. So let your mind process them unconsciously, that's what intuition is. Be ready to get some insights whenever they pop up and to welcome them with gratefulness.
Now carry on. Pick up another planet. Let's say this one is a slower one. If it is conjunct to the Moon and Mercury, then the energy of this slower planet becomes much more personal than it would otherwise be. So invite into your relaxed mind (take another deep breath or two) to remember the meanings of this planet in this sign and house, and also the meanings of this planet conjunct Mercury and the Moon.
At that stage, maybe you will start to find interpretations that are very similar to things you have already found. Let me give you an example to be clear: Suppose your stellium is in Capricorn. You have interpreted Moon in Capricorn, Mercury in Capricorn..... and now your third planet is, let's say Saturn. Moon and Mercury conjunct Saturn in Capricorn, it's like Moon and Mercury in Super Capricorn.... So there is nothing new here, only a more important emphasis on Capricorn.
Take another deep breath and look at the other planets if there are more. Let's say Neptune is here as well, conjunct Moon-Mercury and Saturn. Start with Moon-Neptune - It's like a Moon in Pisces. Another way to think of it is to wonder: what do these two planet energies have in common?
Moon and Neptune are both water, which means sensitivity, imagination, empathy, inspiration... so add these traits as an extra layer to what you have already found. As it is happening in Capricorn in my example, the person is likely to be more particularly sensitive to Capricorn things, to dream of Capricorn stuff... So if Moon-Neptune is romantic, they will cry at the words "happily ever after" because Capricorn loves long term, and whatever resists the assaults of time. If the person is intuitive and psychic, in Capricorn these qualities may be useful to reach their goals..........
Same thing with Mercury......... think of yourself as a writer creating a character. There are several facets to its personality. It doesn't has to be all perfectly integrated, there are just several facets.
At that stage, well I don't want to tell you what to do, as we all have to find out what works for us, but if I don't have a gardening job to go to, I may go for a walk and let myself daydream, or just forget and let the subconscious work on its own.
Sometimes, just looking somewhere else gives the Great Spirit - the opportunity to show you something that's just relevant.