You can birth notify when they are four weeks old, but there's no problem in waiting until their ears are big enough to tattoo any that you are going to register as pedigree.
I do mine just before weaning (normally 8 weeks). There is nothing stopping you advertising your pigs for sale in advance.
Only tattoo the ones you are going to register - IF any meet all the requirements of the breed standard, of course.
Is there anyone living within easy reach who might be able to show you how they tattoo their own pigs? Always easier when someone shows you.
Good luck. Liz