⬢Mount the "lower" fitting to the top of the heatsink. Screw it all the way, but don't tighten it. As soon as you reach the end of the thread, release it by a full turn (360 °).
Pay attention to the correct orientation of the fitting. Insert the coarse-threaded side into the heatsink.
⬢Push the heaterblock up against the fitting with a reasonable force. No need to bend the entire axis. This will pre-stress the PTFE tube inside. Now, it is important to keep the slight upwards pressure.
⬢Make sure the heaterblock is aligned with the heatsink. If not, realign it, but keep the pressure.
⬢While holding the heaterblock from below (pressing the tube inside), tighten all three grub screws.
⬢Now, you can release the heaterblock. Make sure it doesn't move. If so, the grub screws were not tightened properly. Release them and return to the beginning of this step.
It is crucial there is no gap between the PTFE tube and other parts of the hotend. This is why we are "pre-stressing" it.