Tattooing stretch marks and scars

I have never had to tattoo a stretch mark before until now. So, I am asking if anyone has tattooed stretch marks or scars. Is there anything that you need to different before you tattoo over a stretch mark or scar? The position of this stretch mark is on the back of the arm and it is numerous places. Advice please?

Make sure the scar is pretty old and flat if possible. make sure people know that it may take a few goes to get the colour in and that it's more painful to tattoo scar tissue than normal skin. I got scratched up pretty bad by an ebay kit kid a while ago, when I went over the tat myself some time later the scarred bits hurt like a mother This was on the forearm which for me is usually fairly painless.