How can i transition my cat to toilet outisde?

For the past year we've had an indoor litter box in sort of like a cubby that she uses but I can't bear the smell. I've built a pergola(not for the cat) and I'd like her to start going outisde ideally in the garden bed but I'm worried about the transition phase. The other thing is I don't know when she toilets. Will she hold it until let outside?

Whats the best way to go about it?