I've read a few topics here and learned about "activators" (press L+R for blahblah, etc). But, how do I add one to a code?

Also, I read a bit about converting codes (for example, from US to EU and vice versa).
Is it true you do it like:
Take two of the same codes, one from US, one from EU.
US/EU code - other region code = difference
And then add or substract the difference from one of the codes to convert it.
Right?

Thanks in advance ^^