I watched a lot of videos, this guy did better than most:
There are three methods I see people use in videos, heat gun, hot water, and steam. I tried the hot water (not boiling) and didn't get great results. I was afraid the heat gun would burn, but it didn't.
You put a spring through the hose to avoid kinking, and a heavy spring holds heat and helps with the melt. This one worked great for the 5/8" heater hose:
https://www.amazon.com/dp/B0BPP2ZLVK
I've done this with a few hoses and go the best results, by far, from the
Goodyear/Continental HYT-Black. It's a commercial grade hose, chosen by ChatGPT as I prioritized durability and formability.
Test on small pieces to get the feel of it. I used the high setting on my heat gun, 3-5 minutes from all sides, as I held or clamped the hose into an exaggerated bend of what I wanted. The hose and spring get very hot and you can field the hose suddenly start to yield. After a short wait I used gloved hands to hold the piece in the exaggerated bend and dunked the hose and spring in water. The hose will spring back slightly, but mostly holds the position where it cools.