That's a hard one to answer with any reliability.
A Friday in mid-July is not the best time to try to find a walk-in site anywhere. Early morning is good for getting ahead of most people, a day or two earlier in the week is better. Look for empty sites with no tag on the post, check the departure dates on any tags that might still happen be posted if nobody is around, watch for people obviously getting ready to leave, ask them if you're not quite sure. Walk or drive around, including on the other side of the bridge. If you find a site that someone will be leaving, go fill out your tag, date it and pay your money. Post that tag as soon as they leave. You just kind of have to get the lay of the land and stake it out.
https://www.nps.gov/yose/planyourvisit/ycreekcamp.htmWith holding your spot, if possible, leave a tent or grill, chairs, something to demonstrate your presence and avoid confusion. You'll also have your own tag that you put on the post that has your name, car license and site numbers, number of nights you'll be there.... When you drop your money in the iron ranger, you fill out both the tag and payment envelope with your info and then you rip the tag off and put it on the post at your site. The other half with the same info is in the box and locked up. Directions on how to do it will be posted by where you pay. Bring a pen.
If in the very rare event that someone pirates your site, talk to the ranger and he/she will sort them out. I don't remember if they have a campground host there or not, but that'll fix the problem too. And with that, since this is the 4th of July, rest assured that the importance of safeguarding the enjoyment of your camping experience is foremost in our priorities as we apply our wonderful system of checks and balances on your behalf, inclusive of in our national parks, and extending in no small way to our LE rangers that understand fully what it takes to deal with the occasional assholes that forgot about the other half of the envelope being locked in a metal box that they don't have the key to.
The road into the campground is six miles, and it's not usually in the best shape. Go slowly. Don't bring your sports car.