I agree with the others, you need to stay out with her until she goes, and take her out when she wakes up, after eating and after any play sessions. I think i was taking mine out every 30mins or so unless asleep. Also had to force him to take naps otherwise he'd get over tired and then misbehave. (a welshie like yours)
Just be patient, and consistent, she will soon learn. Mopping up pee is just part of the fun. I agree putting a command on the toileting action is a huge help in the future (ie you can ensure they have been before you start to train or work them, so that you know they will be able to give you their complete attention)