Once you have your Mailchimp or Constant Contact account connected in Form Settings (if you don't see the MailChimp and/or Constant Contact option in Forms, Settings contact us to enable it for you), the next step depends on how many lists you want to allow someone to sign up for.
If you only have one list, skip to Step 2.
If you have multiple lists, e.g. Monthly Newsletter, Events, Advocacy etc., you need to put checkboxes on your form so that the person signing up can indicate their choice(s).
Step 1. Multiple Lists
Then you will need to create a Constant Contact or MailChimp 'feed' on the form for EACH list, which will send the form info to Constant Contact.
On your form go to Settings, Constant Contact or MailChimp Feeds, and Add New. Give your feed a name (for instance the name of the list) and then choose the correct list from the drop down.
Step 2. Add Feed and Choose List
When you do that the merge fields shown will appear. Match the form field on your form to the MailChimp or Constant Contact field name.
Finally, scroll all the way to the bottom and set Conditional Logic to only process the feed if the choice the subscriber made matches this list.
Step 3. Add Conditional Logic to Feed
You should test that you've set up the form correctly by submitting a test entry, then checking in Mailchimp or Constant Contact to make sure your test was added as a contact to the list you chose.