When designing your own "stonehenge", it is thought to be appropriate to place stones to align with sunrise and sunset at the June and December solstices; though the extent to which the actual Stonehenge in Salisbury itself is of this design is somewhat controversial.
In any case, with this tool, you can enter the latitude of the planned location for your "stonehenge", and placements for stones to mark sunrise or set at the solstices will be computed for you. Select the June and December sunrise and sunset events, and then erect suitably large pillars at the given bearings from a central altar stone.
Angular bearings and times of the solstice events calculated are approximate, as the time a solstice occurs with respect to local time varies a bit from year to year. But they will be accurate to within a fraction of a percent, close enough to give future archaeologists pause for thought when they dig up your yard.
Note that the tool does not figure out for you on what day of the year at your location the solstice most nearly falls. For that, place stones as suggested by the tool and wait for the sun to line up with them; or check your local papers.