In a food processor, blend the almonds until you have a fine flour.
Add in the erythritol and blend quickly just to combine.
Add the stevia, almond extract and 1/2 tsp. water.
Blend well for at least 1 full minute.
If, when you pinch the dough together it's still crumbly, add another 1/2 tsp. and blend for another full minute.
Repeat this process until you can form the marzipan into a soft, almost sticky ball of dough. Make sure it's not so sticky that big bits sticks to your hands. If you get too much water in there, add 1 tbsp at a time of the erythritol until it balances out, blending between each addition.
Roll the marzipan or press it into molds and place in the fridge. If you press into a mold, thing swill go quicker if you place it in the freezer for a couple of hours.
Then (if you want it to be chocolate covered) you can melt a bar of low carb chocolate in the microwave. First for 1 minute, then at 30 second intervals, stirring each time until the chocolate is fully melted and drizzle over your marzipan.
Notes
Please note that the nutrition data below is a ballpark figure. Exact data is not possible.