magical tattoo artist

Living tattoos are imbued with magic. You can only tattoo spells up the the level you are (e.g. if you are level 7, you can only tattoo spells up to level 7). Tattooing a spell requires the use of one of your spell slots equal to the spell's level. Cantrips do not require the use of a spell slot. The time it takes to tattoo is based off of the size and detail of the design, plus several more hours for living tattoos. You cannot magically tattoo yourself.

Regardless of what class you are, you get a flash book which is similar to a spellbook. The flash book includes spells you can tattoo, but not cast. You can learn new spells outside your class list by training with another artist who knows the spell or if a client brings you the spell and all required components. Before being able to tattoo a new spell, you must spend several hours to learn it, though you only have to do this once. 

The majority of living tattoos are simply using minor illusion which causes the tattoo to shimmer and move as if alive. Most living tattoos are designed to reflect the spell they represent. 

Passive cantrip tattoos (such as minor illusion) are active at all times unless the wearer wishes it to stop. This requires no effort or action. Active spells, including cantrips (such as poison spray), require an action to activate them. Cantrip tattoos can be activated as many times as desired. Higher level tattoos can only be activated once per long rest, but they do not count towards the wearer's used spell slots. 

Living tattoo examples:

dancing lights (cantrip): a faint shimmer lurks beneath the tattoo at all times. You can pull the light from your skin and use it according to the spell's rules.

arcane lock (2nd level): You can lock any opening according to the spell's rules. This does not count towards your used spell slots.