There is a good fridging guide in the stickies - give it a read.
The basics are easy - set your fridge to 5-8C, put your axie in a tub of clean dechlorinated water, put it in the fridge, cover it with a tea towel to keep it dark. Fill a large bottle of water up with dechlorinated water and put that in the fridge too, so you can change the water later. Leave your axie in for 24 hours - it won't need feeding and it may be quite lethargic - this is normal. But it should poop. Change the water after 24 hours to prevent ammonia build up. You can leave an axie in the fridge for a week - it won't hurt it. When you want to put your axie back it the tank just leave the tub (covered) at room temp to warm up gradually, then put it back in the tank.
But 8 isn't old for an axie, it's just middle aged. Axies live about 15 years if looked after properly.