Congrats on the weight loss, great achievement.
Heat oven to 80°C.
Remove all straps.
Put guard on a oven tray - do one at a time, I also like to line the tray with grease proof paper.
Place in the middle of the oven, set a timer and heat for 10 mins.
Remove and immediately shape the guard to suit and fix with the straps - do it a little tighter, as I found they relax a little once cooled down.
Take off after 15 mins and repeat for second guard - I put the second in the oven, when removing the first to wear/shape.