The good thing with celandine tattoos is, you see the results right away.
The bad thing is, it's even more fleeting than a dandelion tattoo: if you're careful it'll stay on your skin for a whole day, if you're not it's gone by evening. A dandelion tattoo will stay on for a whole week, if you're careful.
How: pick the top of a leaf- or flowerstalk of celandine (Chelidonium majus), wait for a drop of yellow sap to form, and use that drop to paint with. When the drop is gone you pick the next 5 cm (2" or so) and paint again, this time using both ends.
Your skin painting is yellow at first, but will turn brown within a few hours. It is in fact easily rubbed off, using warm water, soap, and a towel, if you feel you've painted too much.
I haven't gotten any on my clothes, so I'm not sure how difficult it is to remove. I do know that the sap of dandelion, which is also used for short-lived tattoos, stains clothing permanently.
Celandine sap is mildly caustic, and has been used (like dandelion sap) to paint warts away. It's not all that toxic, but I wouldn't want to bathe in pure celandine sap.
That being said, have fun!
Related entry: Chelidonium again