Your butt qualifies to be branded thus, if:
you have attained First Slave in one or more LULU Signature Gags (tatt number will reflect this number)
To get yours,
If you have a Mesh Body (e.g. Maitreya, SLink etc.) a regular Tattoo will not work. You need Omega Advanced Appliers.
These are the steps:
A menu will come up asking which layer you wish to apply to. Just select Tattoo.
That's it!