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.

Magento 2 Admin Panel,CustomersThen find the customer you are interested in and press Login as Customer button.

Magento 2 Login As Customer,Customers

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.

Magento 2 Login as Customer,Edit Page

Admin Order Grid

Please go to Admin Panel > Sales > Orders.

Magento 2 Login as Customer,Admin Order Grid

Afterward, find the order you are interested in and press Login as Customer button.

Magento 2 Login as Customer,Admin Order Grid

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.

Magento 2 Login as Customer,Admin Order Edit Page

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.

Magento 2 Login as Customer,Invoices

Find the invoice you are interested in and press View button.

Magento 2 Login as Customer,Invoices

Find the Login AS Customer button.

Magento 2 Login as Customer,Invoices Edit Page

You can login as a customer from the Shipment and Credit Memos pages as well, following the same operations described above.