There is a special sealing washer on that 24mm bolt you need to replace also. Both o-ring and sealing washer should cost no more than $ 10 USD from your local, friendly stealership.
1. Drain at about 1 gal of coolant into clean container for reuse.
2. With skidplate removed and working on your back,
3. With pliers, enlarge and move back the spring clips that hold the coolant rubber hoses onto the oil cooler.
4. Remove 24mm bolt and carefully pull off oil cooler.
5. Lube o-ring in a thick grease to hold it in place on its groove cut into oil cooler. Replace sealing washer on bolt.
6. Carefully match up two pronged key on cooler to its seam on eng block.
7. Tighten bolt and replace coolant hoses and coolant.
8. Chug a few beers.:reddance: