Put them where you'd find them more practical? The bridges are a way to get you from the north side to the south side and vice-versa more quickly. If you're often on that peninsula on the left and find yourself annoyed by having to walk all the way to that bridge to get to your house, you can put it there. You can also put it more to the right, if it makes it easier for you to get from the beach to Re-Tail. It just depends on what you find easier when walking around your town.
You can also use bridges (like other PWP) to prevent villagers from moving into certain spots. If that's what you're trying to do, that also depends on where you don't want villagers to set down their houses.
Obviously, it would be silly for me to presume best placement of bridges, since your daily habits are different from mine.
That said, I believe David and Capella have equally decent suggestions because both provide decent spacing between the bridges, optimizing the area covered.
Relatedly, David's seems to provide less spacing, but the inverted-U shape of the river means that the bridges are providing a lot more convenience than they otherwise would.
This seems intuitive enough, and I'm likely just being Captain Obvious here.