You need to create a numbered directory for your b24j skin - if you don't already have any, start with '1', i.e. "b24j1". This directory goes in the skins folder of your Aces High II directory.
put your b24j-1.bmp file there, as well as a copy of "material.txt" and create a text file called b24j.txt and put your identifying info in there - for example '324BG/333BS by SkyChimp'.
Then start up AH2, go offline, and you should see this description in the skin selection drop down list for the b24j.
Good Luck, post a screenshot for us when you get it finished up!