Go to Configuration → Approval Process → select the particular approval process → edit
In the Approval step(s) field, click the + (plus) button in the preferred area.
The multi-approval can be configured in either one step or many steps:

Go to Configuration → Approval Process → select the particular approval process → edit
In the Approval step(s) field, click the + (plus) button in the preferred area.
The multi-approval can be configured in either one step or many steps: