To make a reservation, visit Recreation.gov and follow the instructions below.
How to Make a Reservation
Still Having Trouble Finding a Reservation?
Another approach is to try to get different campsites each night (rather than one campsite for your entire stay). To do this, change the arrival date to your first desired night, the departure date to the next day, and specify that you're flexible for the next two weeks. With this method, you can search campgrounds in and near Yosemite all at once. Once you've chosen a specific campground, click on the date range availability choice (see below) to see if some of your nights are available. If you don't find availability using these methods, it's worth checking back as often as possible: cancellations will appear on the website immediately.
It's common for no reservations to be available from spring through fall. If you are unable to find a reservation, you can try camping without a reservation.