If you are using mussels in the recipe, you can keep them in the shell, especially if they are small but if you want to add them without the shell, simply steam them for only about 3 minutes until they open up. They can finish cooking in the broth when you addd them with the other seafood. This is my preferred method because I don't want to leave mussels that don't open in my chowder. You should always discard a mussel that does not open when cooked.
Some people prefer to add potatoes to seafood chowder. I sometimes do if serving it as a complete meal but mostly don't if I'm serving it as an appetizer. If using potatoes, choose yellow waxy potatoes and not starchier potatoes like russets or blue potatoes. Those will likely add additional thickening to the soup. I parboil the potatoes for 10 minutes, let them cool for a few minutes before dicing them into bite sized pieces. I add the potatoes during the last 10 minutes of the simmering of the broth.
If using fresh herbs in the broth instead of dried you can double the amount. (Except for bay leaves)