You click immediately after she’s done the trick (or, for shaping, part of the trick). Then you give the treat as quickly as you can. Then you praise.
But first of all, you teach her that a click means a reward. In your case I’d use her normal pellets. Don’t serve her her full daily ration of pellets in her bowl, save a portion of it for training. Because rabbits are always hungry (and they are supposed to be), a pellet will be seen as a reward. The reward needs to be small so it can be eaten quickly.
Your first exercise should be to teach her what the click means. Decide on sth really simple for her to do, something she’ll do “of herself”, like look at you. When she does it, click and reward. When she does the same thing again, click and reward. And repeat. The lesson learned is supposed to be click means reward. Every click must result in a reward, even if you’ve happened to click when you shouldn’t have. Do short sessions, or she’ll get too many treats.
This principle goes for clicker training any animal, even chickens. I’ve clicker trained my dog.
As for veggies: At 12 weeks you can start intriducing veggies. Romaine lettuce is a good starter-veg. Herbs are good too, for example cilantro, dill, parsley. Some buns like sage, oregano and basil, but all buns are different so not all buns like exactly the same things. Here’s info on a healthy bunny diet, with a link to the House Rabbit Siciety’s list of safe greens and fruit: https://binkybunny.com/BUNNYINFO/tabid/53/CategoryID/2/PID/940/Default.aspx
Your bun will of course like clementine, but fruit must be given sparingly. Berries are lower in sugar, so they are better.