[*]Log into your ACP
[*]Click Smilies
[*]Click + Add New Smilie
[*]Give the smiley a title, such as "Whistle"
[*]Set the Text to Replace, which could be anything from :-0 to ::Whistle::
[*]Note: Whatever value is entered into this field will be replaced by your custom smiley in post content.
Image Replacement URL (the location of your smiley sprite sheet or image).
[*]default sprite location is: styles/default/xenforo/xenforo-smilies-sprite.png
[*]Note: You can also upload a single smiley. Simply enter the URL of the smiley's location in your database, then un-check Sprite Mode, save and you're done.
Sprite Mode is enabled.
[*]Leave Sprite Dimension untouched.. this is just your smiley's dimensions (18 x 18). Only adjust this value if you're uploading a large custom smiley (set it to that smileys dimensions when you do).
[*]The Sprite Position values are the X and Y on the sprite sheet. X is horizontal, while Y is vertical. Smileys on the X axis are 20 pixels away from the next, while smileys on the Y axis are 21 pixels away from the next... so for example.. 0, 0 would be the top left corner, and -80, -126 would be the bottom right corner (which would be the smiley for 'Whistle').
I've created an X, Y axis chart for you, using the default sprite sheet. Hope this helps!