You must rule out medical problems first.
Here is a list I compiled previously. Some of the items may not apply.
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You might try an integrated approach using multiple options from the list below on a consistent basis:
* start with crating, using a medium large crate (30" long is big enough for cat and a litterbox) until progress. She probably won't go where she'd have to lie in it ... but if she does, something is wrong and it may be medical.
* tarps and newspapers may be easier on clean up if you don't crate her
* feeding on a schedule and then placing her in the litterbox (maybe in a crate) within 15-20 minutes after eating, until she goes, then praising and letting her out.
* Feliway/Comfort Zone - friendly facial pheromones which reduce stress and may reduce marking behavior. Cheapest prices are usually on the net (ex
http://www.LambertVetSupply.com ) I've used it and it helps.
* Cat Attract litter - supposed to help attract cat to litter box
* rule out arthritis or constipation - pain causes box avoidance
* watch if the other cats ever attack her in the box and if so, you may need to keep her separated from them. I've got one the others attack, completely unprovoked.
* littebox should be 1.5 times length of cat, litter cleaned frequently, but avoid really strong smelling cleaners as cats may avoid them due to the scent.
* you've got to get residual odors out from areas which were 'nailed' - Nature's Miracle is one option and has good reviews. It _can_ require repeated applications.
* regular play sessions of about 15 minutes to reduce stress and promote bond with you
* reward desirable behaviors - if you see her use the box, praise and treat her
* put unpleasant textures in places she has peed to encourage avoidance
* read some of this author's books:
http://www.catbehaviorassociates.com/ she's pretty good - ex "Starting from Scratch"
Good luck.