You probably have heard that the minimum gain is off the ends of the wires, and the maximum gain is at 90 degrees to the wires. This means you will need to have the wires run at 90 degrees to the direction you want to aim your dipole.
However, this still does not answer the original question. The direction you want to aim your dipole depends on who you want to hear, and where you are.
The process of aiming a dipole is the same as aiming any antenna, so this applies to all antennas that have some directional abilities.
I like to make contacts in Europe and I live in the northwest part of the
U. S. A. which puts Europe at a great circle angle of around 30 degrees. That may seem like a very wrong angle, but using the great circle calculations will give an angle of about 30 degrees. Use the great circle method anytime you are interested in aiming any antenna.
A great circle is the shortest distance between your station and the station you want to hear.
Probably the easiest way to find the great circle direction is to have globe and a string. Put the end of the string on your location, and the other end of the string on the location you would like to talk with.
Pull the string tight enough so there are no kinks in it, and so it is as straight as it will get on a curved surface.
With a flexable protractor, measure the angle between the nearest line of latitude and the string.
The next easiest method is to buy a great circle map that is centered on your location. You can get these from ARRL or you can look in the QST magizine for some advertisements.
Finally, for those who would like to do it with a computer, follow the directions below.
Step 1) Find the Latitude and Longitude of the world city.
Step 2) Use a great circle calculator.
To find the Latitude and Longitude of a world city, try any of these addresses. If you can not find the city or country on one of these, another one is likely to have it.
See if you can find London, England on each of these web sites.
2) Baha'i Computer Communication Association
3) Astrology Information
London England has a Latitude of about 51 degrees and 30 minutes N and it has a longitude of around 0 degrees and 10 minutes West. The web sites may not agree with each other, but they will be close enough.
To find the great circle direction, I like to use the calculators found at the following addresses. All are good so find the one you like the best.
1) Ed Williams' Aviation Page
2) Andrew Gray's excelent map shows the cities and countries and you choose them with your mouse.
3) Satellite Signals LTD.
4) Movable Type Scripts shows you a map to see if you typed the correct information ( but it is not a great circle map.)
To get the bearing, you need to drop down more and press the "Calculate initial Bearing" button.
How close do you have to be to the angle?
A dipole has a very wide lobe so you only need to be close, but a yagi has a much narrower lobe so you should try to be as close as possible.
Now that you have the direction to the country or city you want to contact, be sure that you run the dipole wire at 90 degrees to that direction. If you are aiming a yagi, aim the boom directly in the calculated direction.
This is the end of this section, but to really learn how to do this, you need to practice finding the bearings for a bunch of locations until you are sure you know how to do it.