Harissa is a chilli paste used in North African cooking and basically consists of ground chilli and garlic.
According to the North African cookbook, before you can make harissa you need to make tabil. Since I hate recipes where you have to make Dish A, B and C before you can actually bloody well start cooking dinner (which is why I don't normally do Japanese cooking), I just buy harissa off the shelf from the local deli. If you don't have harissa, a good substitute is sambal olek - a Malaysian chilli sauce which tastes similar to harissa. Sambal olek is sold in the Asian section of supermarkets as well as in Asian grocery stores.
Here are directions for making your own harissa.
1/2 cup ground coriander
1 tablespoon ground caraway seeds
11/2 tablespoon garlic powder
1 tablespoon chilli powder
Mix all the spices together and store in an airtight container. Use within 2 months.
6-8 dried red chillies
1/2 teaspoon sea salt
1 teaspoon of tabil
1/2 extra-virgin olive oil
Chop up the chillies with scissors and soak in hot water until soft. Drain them and use a paper towel to remove excess water. Put the chillies and other ingredients into a blender and mix into a paste. Place in a glass jar, cover with a thin layer of olive oiil and seal tightly. Refrigerate for up to 3-4 weeks.