Making reservations is pretty easy but it does take time for a popular place like Yosemite. The hardest thing to do is to make your commitment early enough. In the summer you need at least two months notice but more time gives you more availabilities. Holliday weekend you need even more time in advance to make the reservation. The Yosemite Valley campgrounds fills up first, then Crane Flat and then Hodgedon Meadows which is another 8 miles or so farther away from the valley but on the same road as Crane Flat. See the Yosemite map. Since both Crane Flat and Hodgedon Meadows are in the hills it is a little harder to find a big enough spot that is level.
Incidently I saw my first Trailmanor at Hodgedon Meadows in 2002 and I was convinced that I had found what we needed.
First you need to pick a range of dates and put them into the internet reservation website.
http://reservations.nps.gov/
Be sure to read all of the instructions and restrictions.
If after you put in your range of dates for the campground of your choice it doesn't accept it, try different dates until you find what you want. Sometimes you have to try the next day if the system isn't working right.
They don't reserve any particular spaces so it is best to get there on the day of your reservation as early as you can so that you have a pick of the available sights. Many people come in really late and don't have a selection.
What is bad about having to make reservations is not knowing what the weather is going to be like. Most of the time between about April and October the weather is very nice. But last year they had a week choked with the smoke of local fires in the valley and a stretch of 10 days of rain in a row. In July and August you have a 90% chance of nice dry weather.
But the higher elevation campgrounds are generally quietier and more peaceful.
Good luck,