First you need to mix records with your friend you can do that on the top floor of the Pokemon center talk to the lady behind the far right counter and choose record corner then when your linked up stand in the colored squares to mix records. Now start searching to find your friend's base when you do find it enter it and check the computer it will ask to register this base you should register the base that way if your friend were to ever change there base and mix records with you again you will still be able to enter the original base and the new base.
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In your secret base.
Unfortunately there is no TV in Ruby/Sapphire/Emerald for a secret base. Many people worldwide will be upset. :(
u cant
u can buy it in slateport
You can't, unless you mean secret base. You have to have a Pokemon with Secret Power. Use Secret Power in front of a tree or an indention in rock.