From the Mail menu bar, select
Mail ▹ Preferences... ▹ Signatures
The preferences window now has three panes. On the left, the signatures are grouped by mail account, plus an All Signatures group at the top. Select that group.
The middle pane now shows the names you gave to all the signatures you've defined. When you select one, the right pane shows a preview of the signature.
In the middle pane, select all the signatures you want to use in a particular account. You can make multiple selections by holding down the shift or command key while clicking. Drag and drop the selection onto the account name in the left pane.
If you want one particular signature to be automatically added to all messages sent from an account, select the account, and then select from the Choose Signature menu at the bottom of the window.