Hey, I signed up just to help; unfortunately I cannot help with a video -yet- but here is the main things you need;
1.) as Smurfkiller said you go into %appdata%, in there you go to .techniclauncher
2.) Within .techniclauncher there are many folders depending on which type of minecraft you installed (aka tekkit, yogscast, ect.) here go to the "tekkit" named folder.
3.) [quite important!!!] When there, you will want to make a backup of the mods folder either in your documents folder or on your desktop (just in case).
4.) once thats done, open the mods folder and you will see a bunch of rar/zip/java files. These are all the mods tekkit runs off on (all 41).
5.) open the files (even the java files) using winrar.
At this point its blind luck if you find the exact thing you want to re-skin, but here are some markers:
Most mods will have either a telltale sign (such as block_machine will be machines and item_ will be items). Everything that starts with gui_ will be the background image of what you interact with within the game (aka crafting tables and furnaces etc.)
IC2 block files can be found here: industrialcraft-2-client_1.97.jar -> ic2 -> sprites (here the machines are titled block_machine.png and block_machine2.png and items are under item_0.png)
Buildcraft textures are here: buildcraft-client-A-core.zip -> net -> minecraft -> src -> buildcraft -> core -> gui -> block_textures.png
Railcraft textures: Railcraft_Client_5.3.3.zip -> railcraft -> textures -> railcraft.png
Redpower World: RedPowerWorld-2.0pr5b2.zip -> eloraam -> world -> world1.png
EE: EE2ClientV1.4.6.5.jar -> ee -> art -> sprites -> eqexterra.png (eqexsheet.png is the item texturepack)
Ironchests: mod_ironchests-client-3.8.0.40.zip -> cpw -> mods -> ironchest -> sprites -> block_textures.png
Weaponmod: Weaponmod.zip -> gui -> weaponmod -> weapons.png (the cannon and other textures are in the other folder titled item -> weaponmod -> ... - I havent touched these in my personal reskinning yet, so I have no idea what is what yet, but it should be straight forward)
There is a bunch of other folders and textures, but I think it shouldn't be that hard to find them. Pretty much you need to look out for folders named either "textures" "sprites" or "gui", and with a little bit of luck you find what you were looking for.
I hope this was helpful.
Happy Skinning!
Decoy out.