First download the Steam Link app from the Play Store or App Store and then open the Steam Link and the Steam client on your PC.
Going to the Steam PC client, you have to open the Steam tab, open the settings, and enable the remote play checkbox.
You have to press the Pair Steam link, button below the list of devices Then you have to open the Steam Link app on your phone and scan the PC, then provide a four-digit PIN.
You have to enter that PIN in the textbox of the device, then a pop-up will appear on the PC You have to click OK and then enter your username which will be shown on the Steam PC client.
After that you have to wait a bit, the Steam link app will check the connection, and then you have to press the start playing button to open the Steam library on the phone.