Trying to 100% the game

I just beat the game, took me 79 hours and I loved it. Now I want to attempt 100% it and want to ask how I should go about it. Do I have to go back to chapter one and play on hard or can I go right into chapter 12 or 8 and play some mini games and use the open world map. I’d prefer not restarting from chapter one atm since I JUST beat it but I do want to attempt 100% completion. What should I do?