Step 1, naturally, is to be in Manhattan.
I'm in New York City today and Scientific American contributing editor Steven Ashley was kind enough to reminded me that my visit is coinciding with Manhattanhenge—a twice-a-year event when the sun lines up with Manhattan's street grid. This year, there will be a Manhattanhenge on May 29/30 and another on July 11/12.
You'll note that Manhattanhenge does not actually occur on the same day as the solstice—when the Sun is at the highest point in the sky and the length of the day begins to get either longer (winter solstice) or shorter (summer solstice). That's because Manhattan's grid is rotated 30 degrees east off of true north, writes Neil deGrasse Tyson on the Hayden Planetarium website. That's enough to make Manhattanhenge less astronomically accurate than Stonehenge. But it's still awfully nifty and is supposed to look really, really cool.
Tonight's event should start around 8:17 pm (Eastern time, of course). Here's Neil deGrasse Tyson's advice on getting a good view:
For best effect, position yourself as far east in Manhattan as possible. But ensure that when you look west across the avenues you can still see New Jersey. Clear cross streets include 14th, 23rd, 34th. 42nd, 57th, and several streets adjacent to them. The Empire State building and the Chrysler building render 34th street and 42nd streets especially striking vistas.
Note that any city crossed by a rectangular grid can identify days where the setting Sun aligns with their streets. But a closer look at such cities around the world shows them to be less than ideal for this purpose. Beyond the grid you need a clear view to the horizon, as Manhattan has across the Hudson River to New Jersey. And tall buildings that line the streets create a vertical channel to frame the setting Sun, creating a striking photographic opportunity.