With practice, and be prepared to spill for quite a while, judging by how many of these exist (quite a few also just recommend to use a funnel):

https://www.wikihow.com/Pee-in-a-Bottle-(Women)

https://www.rei.com/blog/hike/girl-talk-peeing-in-the-backco...

https://www.wikihow.com/Pee-in-a-Bottle

https://www.tripsavvy.com/can-women-pee-in-a-bottle-backpack...

https://www.ozmo.io/how-can-a-girl-pee-in-a-water-bottle/

(and many, many more)

It certainly doesn't seem anywhere near as easy as just [using] a water bottle.