I want to attempt making marzipan, and the recipe I'm considering calls for 200 grams ground almonds (link to full recipe). How fine should the almonds be ground? Is it something that I can accomplish at home (I have a food processor or a coffee grinder), or should I be looking for a store-bought product to get the right consistency (if so, what's it called)?
The recipe calls for ground almonds. This can be done in a food processor. You may wish to remove the almonds' skins beforehand, and you also may wish to toast them. Both of those are very common, even though neither is essential. With this particular recipe, I'd be inclined to toast them but leave the skins on.
Once you've made those decisions, measure out the sugar called for in the recipe. Set it aside. Then place the almonds in the processor and add two or three heaping tablespoons of the measured sugar. Pulse until you reach a grind that resembles a meal, or very course flour.
The finer you grind the almonds, the more carefully you must watch them. Eventually, the almonds will begin to turn to a paste. The sugar is helping to prevent it, and that's helping you to get a finer grind. However, overprocess and the almonds will turn to paste. If you're carefully watching for it, though, you'll see it begin to happen before it goes too far.
A processor can be used if it has sharp blades and is decently powered. Drop almonds into boiling water less than 60 seconds then introduce to an ice bath immediately. the almonds will literally pop out of skins when squeezed. DE-skin all almonds and dry thoroughly with a tea towel or paper towels. Add dried almonds to processor and pulse until a very small grind is achieved then add powdered sugar slowly while pulsing until desired paste is attained. I use a masticating juicer to make a smooth almond butter before putting it in a processor to blend in the sugar or sweetener of choice.