Determine which direction to transpose the piece. Depending on the situation or your personal preference, you may want to transpose the piece up a minor third or down a major sixth. If the piece is an accompaniment to a vocalist, ask if she has a preference.
Transpose all notes by the appropriate interval in the same direction, such as a minor third up or major sixth down. C, D, E, F, G, A, and B will transpose to E-flat, F, G, A-flat, B-flat, C, and D respectively.
Copy all relevant musical information if you are writing this anew. Relevant musical information includes rhythm, phrase markings, tempo markings, descriptive words and anything that you deem important to the correct interpretation of the piece.
Play through the piece in both keys, a few measures at a time, to check for any audible errors. If necessary, correct the errors.