How do I sign a customer up to one of my SaaS plans?

There are three main ways.

1. Billing page: give the customer a login. They go to Settings, then Billing, and pick the plan they want.
2. From the contact record: open the customer's contact, go to the Billing tab and click the plus button to add a plan. If you want the customer to enter their own card, select free of charge, pick the package and click Complete purchase. The customer gets an invitation email saying their account is ready. They activate it, create a password, log in, click Settings, then Billing, and pick or pay for the plan.
3. Payment form: build a payment form and put it on your website, or send the customer the link to the form.

Working from the contact record keeps their calls, notes, transcripts and call summaries on the same record, and the Login as button lets you work inside their account. You can also create a trial pricing option on a plan, for example a one day or seven day trial, for customers who sign up and pick their own plan.