We probably need a little info about how your particular boat and topping lift are set up.
@rgranger linked a quick tutorial, but boats can be set up differently.
On my O’Day 322, the topping lift was rope, went from the back of the boom, up to the top of the mast, over a sheave, down the inside of the mast, and it exited 5 feet above the deck. There is a cleat ton the mast to secure the line.
I almost never messed with the topping lift, it kept the boom out of the cockpit when the sail was down. Other than that, I never really messed with it. I tried to replace the old crusty rope and realize the sheave is broken…so I went with a Seldon rod kicker. No more topping lift.
I will replace the sheave when I pull the mast for some other work in the next year…but for now, not really needed.
Other boats have the TL fixed at both the top of the mast and the end of the boom…other have adjustment tackle at the end of the boom… still others use a pig tail tied to the backstay to hold up the boom when the sail is down.
So, if you have a photo, we can give a lot better advice.
Good luck with your O’Day!
Greg