Are you her mother? Step mother? Teacher?
(It actually does make a difference in how you look at this).
Her stepmother- she is having issues in general as her mother just had a baby with her new husband.
maybe a step stool might help if she is not tall enough to stand up from the seat w/out getting poop on the seat. have you tried wet wipes along w/ or in stead of just toilet paper? There are Kandoo wipes & cottonelle has them as well that are flushable. Then when she's done w/ the potty then she needs to check & see if there's anything on the toilet. if there is, then there's Clorox, or Green Works wipes that she can use to clean up after herself. just some ideas. good luck & Happy New Year!!!!
It could be attention seeking but if she's always done this from being toilet trained then it could just be that she needs to take more care and pay attention to what she is doing. When she goes without making a mess really make a fuss of her so she can see the benefit of taking more care. Also make sure she cleans her own mess if she leaves it - be matter of fact about it don't get too angry (which can encourage attention seeking) but make sure she cleans it up. This will give her practice and also if it's just laziness she will soon learn that not making a mess in the first place is much easier!