To install this skin simply find your user skins folder inside the War Thunder folder wherever that may be for example: C:\Program Files (x86)\Steam\SteamApps\common\War Thunder\UserSkins. 
Take the folder called template_mosquito-fb-mk6, copy the entire thing into your UserSkins folder, go onto the game and click customise and click the refresh symbol next to the wand, this should add and arrow
next to 'disabled' on the user skin tab, click that and the skin beneath called 'user: template_mosquito_fb_mk6' will be the new skin.