I just wrote this on Reddit, and it’s basically an instant blogpost:

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[…] I have a lot of success with a Pezzetti “6 cup” moka pot (which takes about 30g of coffee and produces about 220ml) and a Alessi 3-cup which produces about half that. Occasionally I have something go wrong like you describe, but rarely nowadays. I make moka on my smallest gas ring. Things I have done and have learned:

• fill with cold water to just below the valve; all this “fill with boiling” stuff sounds like an invitation to burnt fingers, and a friend from Napoli would probably consider it heresy.
• insert basket; the neck of the basket will make the water rise higher, so make sure you didn’t overfill with water to allow for that.
• gently SPOON ground coffee into the basket, never tamping, but mostly-filling the base and building a gentle parabolic mound that rises slightly above the edge of the basket.
• My goal is to build a soft mound which I imagine will, upon squashing, lightly fill the basket. I must admit that I cheated in part by once loading it gently “flat” and then weighing the beans, to give me a rule of thumb for how many beans I should grind.
• the grind? Generally 8-out-of-40 on my Baratza, about 3 to 5 stops coarser than espresso. Generally a filter roast rather than espresso, though both work.
• screw the lid down GENTLY to compress the mound.
• put onto smallest gas ring on a medium-fierce heat, and wait until the coffee starts and then finishes flowing.

I’ve had disasters from:

• too much water
• too little water
• too fine grind (choked, which I suspect is your most likely problem)
• too coarse grind (dishwater)
• accidental tamping / compression / overloading of the grinds during loading; this isn’t like espresso tamping, it’s a more delicate process like spooning whipped cream onto a pudding for serving, and trying to guess the right amount to avoid an explosion when the lid screws down.

Also: the grind varies with bean. Alas.

I have no idea why a 6-cup is advertised thusly; my Pezzetti takes 25-to-32g of softly-heaped grind and makes enough coffee to split between me and my partner, in about 1/3rd of a mug apiece. That said: we’re coffee fiends.

Best of luck. If you want, DM me, and I’ll photograph my process.

ps: if it’s an aluminium pot, don’t dishwash it, it will turn grey.