Well the key is not to lose your temper or use forms of punishment as such. The peeing mechanism, behaviour and restraint takes time to mature, a process that can be set back by stress.
Step one, lets get excited! First we have to pick a form of reward. I like to use two forms.
First is a more frequent reward system such as stickers going up a reward ladder on a poster for every day she is pee free. Although maybe start with every morning, afternoon, and evening she goes pee free then swap posters and try for every day she goes pee free after that. That way she gets to understand what it is like to get a sticker and be adored during the process.
Then set the big goal, going one month without a single mistake! Always promise her something really special. This could be cheap such as having all her friends over to play at once, or a visit to her grandmas or maybe something more expensive like a bike! Only you will know what she would consider a reward worth trying for and what is feasible for you to honour.
Now that we have got the reward out the way, we need to know how to deal when she does pee herself. Take her to the shower and wash her bottom end. Kids don't often like water so it is a form of punishment. The key is not to treat it as a form of punishment as such but a necessary response.
What I mean is don't shout or tell her she has been naughty. If she pees her pants then she must be cleaned. Action and reaction. Equally don't make it all smiles and excitement. Be cool and calm and methodical even clinical in the way you apply water around her legs, drying her off and sending her on her way. If she seem to not care that she is being stripped off and cleaned with water then, while not using anger to make the event unpleasant, maybe make the water a little colder (within sensible reason) if needed to make the even a tiny amount unpleasant. What you want is complaints but not tears. Make it's an event that she understands is her own doing and one that she thinks is best avoided (not just you) . She should understand it is a necessary response as she is now dirty and a consistent response that will always happen if she pees herself. Hence make sure this is implemented in a consistent manner, consistency in response is key.
One last point about the reward chart, whether you are working on morning/afternoon/evening time periods for rewards or the daily rewards, always bring her to the chart whether she has won a sticker or not. Then announce whether she has won a sticker. If she has not won a sticker explain to her the next available chance to win a sticker and what she must do to achieve the goal. The reward chart is as useful when used to reward as it is to emphasis that there is no reward that day. On times when no sticker is to be issued she still must come to the poster and find out the result. During this sticker-less phase do not punish her for failing to get a sticker nor express any angry about her failing to get a sticker. Express more a desire and belief that she will achieve the goal next time.