If you'd like to import an old Borderlands save into the current-gen version of Borderlands: Game of the Year Edition (AKA Borderlands: Game of the Year Edition Enhanced on PC), it's a pretty simple process. We've broken down the process based on Platform.
Note: Vault Hunters in the Japan region are unable to export their saves at this time.
PC
This is relatively straightforward. If you have a saved Steam file on your computer, install Borderlands GOTY Enhanced and start the game.
On the main menu, select the "Import Character" option.
From there, select the character you want to import. Then hit Enter. The game will take care of things from there.
Consoles:
On previous-generation consoles (PlayStation 3 and Xbox 360), you'll need to upload your save file. To do this, select "Export Character" from the main menu and follow the ensuing instructions.
NOTE: You can only upload one character save file at a time, and the save file won't transfer back from current-gen to old-gen. Also, you can only transfer saves within the same console family (PS3 to PS4, 360 to XB1).
After that, fire up your copy of Borderlands: Game of the Year Edition on PS4/XB1, and then select "Import Character" from the main menu.