Is it bonus money that you can play any amount you want, or is it bonus bets? My guess is it’s the latter (5 bets @ $50 each or whatever)
I guess it depends on what you’re trying to accomplish (quick rollover to build a bankroll, or just have fun). This time of year I’d probably use them on the players championship and some March madness winners or something or get down some MLB season long bets
Also, let’s say you’re using a $50 free bet on a -500 team or something, pretty sure you only get credited the winning amount ($10 or whatever) and not your bet. So they’re usually better to use on longer odds bets
Edit: it should give you the option to use one of the bets in your bet slip. Like bet Tarik Skubal for Cy Young and then there should be a prompt to use one of them bonus bets