First of all make sure have Login As Customer Extension installed on your Magento shop, which enables you to login as a customer from several places such as:
Admin Customer Grid
Please, go to Admin Panel > Customers > All Customers.
Then find the customer you are interested in and press Login as Customer button.
Admin Customer Edit Page
Follow the same directions as previously, but instead of pressing the Login as Customer, choose an Edit button. On a customer edit page find Login As Customer button.
Admin Order Grid
Please go to Admin Panel > Sales > Orders.
Afterward, find the order you are interested in and press Login as Customer button.
Orders View Page
Please, follow the same directions as during admin order grid. Find a View button and then on order view page press the Login as Customer button.
It is important to remember that to login as a customer from the order's pages, the order should be submited by a customer not by a guest.
Invoices Edit Page
Please go to Admin Panel > Sales > Invoices.
Find the invoice you are interested in and press View button.
Find the Login AS Customer button.
You can login as a customer from the Shipment and Credit Memos pages as well, following the same operations described above.