Create a short sentence that describes the sigil's intended purpose. Remember to be forceful. It's "I will win the tournament," not "I want to win the tournament." After you have a purpose, write the letters in alphabetical order. Now, cross out all of the repeat letters so each letter only appears once. Take the letters and stick them together so that they become a single strange shape. Try to over-lap lines so that the letters are less obvious. For example P and L can share the same vertical line so that at the top you have the curve of the P and at the bottom you have the foot of the L.
Write the sigil on a fresh paper (or parchment or whatever else you're going to use). Destroy all of the papers you used in the creation of the sigil. You may wish to write your sigil using blood or something related to the nature of the ritual (Oils associated with Jupiter if the ritual is for wealth, etc.). Those who can't handle bleeding that much can prick a finger and trace over the sigil. This won't be enough blood to actually draw the sigil real well on it's own, but it will help charge the sigil.
The sigil is already charged from the creation of it, but if you wish to charge it further you can cause extremes of pain or pleasure while meditating upon the sigil to fill the sigil with more energy.
After you are done, destroy the sigil and make no effort to remember what it looks like. You want the sigil completely out of the conscious mind so that the subconscious can get to work with it.