First of all download and read the attachment to the first post in this thread:
Art Mods as it contains all the information you need.
If you have the artwork from another mod it will save a lot of effort (on your part, I hope you will give credit to the actual artist [;)]). You mention "side art sheets". If you are talking about A_PlaneSide.bmp (or J_PlaneSide.bmp), I wouldn't bother with them. [:(] They are the defaults and if you edit them you have to make sure you don't stomp over previous changes others have made.
A far more elegant solution (as described in the "Single aircraft images" section of the Art Modifications document above) is to apply your updated artwork as a single aircraft image which overwrites a single cell/slot of the default artwork grid. To uninstall simply delete the additional artwork which will unmask the default artwork and make it visible again. (No editing...[:)])
In a nutshell, take your updated 150 x 60 pixel side art image (which you may have to extract from you mod "grid" artwork) and save it as "A_PlaneSidexxxx.bmp" or ("J_PlaneSidexxxx.bmp") where xxxx is the 4 digit aircraft slot number (from the editor). Allied aircraft are stored in “art\AlliedPlanes” and Japanese aircraft in “art\JapPlanes” folders. Your custom artwork will now be displayed in game for the aircraft with slot number xxxx.
A similar method is used for the top art and alpha mask - See the Art Modifications document above.
I won't go into too much detail here but you can also make your custom artwork 'rotate' by game date if you format your artwork as per the Art Modifications document above.
It's actually very simple to do so don't be afraid to give it a go!! (see uninstall above).
Best of luck,