I made a video on how to selfboost with Killjoy and her Turret. (with examples)

Video: https://youtu.be/2xAmxu-H2kM

explanation: Selfboosting in VALORANT kinda works like 3man boosting in CS:GO. You can't keep standing on top of the turret, therefore you have to find an angle on the map where you can get stuck between the turret and the wall (e.g. the tower next to showers on bind). Then you have to position yourself with your back to the wall. Afterwards, you take out the turret and look down (you have to practice the right angle for the Turret, so it presses you into the angle of the wall). Lastly you have to crouch-jump and press "S" simultaneously and place the Turret at the peak of your jump. Now that you have gained some centimeters, you can crouch jump backwards again, to get on top of the box.

Unfortunately there are not many spots, but I have tried to show every spot I could find in my video. If you know any other useful position, feel free to post it here or comment under my video.

I hope this was well explained:)