To build a backup Jenkins node, you can follow these steps:

1. Open the Jenkins web interface and click on “Manage Jenkins” in the left menu.
2. Select “Manage Nodes” from the menu on the left.
3. Click on the “New Node” button to create a new node.
4. Give your node a name and select “Dumb Slave” as the node type.
5. In the “Usage” section, select “Only build jobs with label expressions matching this node” and enter a label for your backup node.
6. In the “Launch method” section, choose how you want to connect to the slave node. You can use “Launch slave agents on Unix machines via ssh” if a slave node is a Unix-based machine, or you can use other methods if the slave node is running on a different operating system.
7. Configure the remaining settings as needed and then click the “Save” button to create the node.

Once the node is created, you can assign your backup node to a specific build by editing the build configuration and specifying the label you assigned to your backup node in the “Restrict where this project can be run” field. This will ensure the build is run on the backup node if the primary node is unavailable.