How do I use Time Capsule to back up more than one Mac?

Set up the second Mac just like you did the first.

Click System Preferences (gear icon on the dock) on the second Mac

Click Time Machine

Make sure Time Machine is ON

Click Add Backup Disk

Select the Time Capsule disk

Click Use Disk

A backup will start in a few minutes

Time Machine will automatically keep separate backups for both Macs