Once a voucher has been sent for your approval, the system will notify you in the Conversation & Notification center. In here you will find a notification regarding the voucher that needs approval.
As you can see from the screen below, you are able open and view the voucher file (often a PDF). Also you can change what job it must be booked on or if the entire or part of the amount most go to a finance account. There are many rules and options that can be applied for setting up your own voucher approval workflow, so please check with your finance team for any specifics.
Once you have approved a voucher it might go to the next approver or, if you are the last one, directly to Project postings in the finance. The list of approvers can be seen under the voucher details.