When you create a base, you also get a QR code associated with this base. All you have to do is to scan the QR code of your friend's base. Then, if you go to the location of your friend's base, you will be able to find it.
For that, your friend needs to share his/her QR code with you. He needs to go to the PC in his base, and to use the "Manage QR Code patterns" option. If he then select "Share a Secret Base", and then "Your Message", he will get a screen with the associated QR Code.
On your side, you also need to go on your base's PC, and select the "Manage QR Code patterns" option. Then use the "Find a Secret Base" option, and you will be able to scan the code.
You might have to wait after midnight for the base to appear on your game.