Replacing the default Kawa-khajiit textures with Immersive Tabaxi Textures is super easy.
All you need to do is rename the Immersive Tabaxi Textures and replace the kawa-khajiit textures with them. For example, if you want to replace the kawa-khajiit face textures with those from Immersive Tabaxi Textures, you just take the Immersive Tabaxi Texture face textures (headtabaxi.dds and headtabaxi_n.dds), rename them kawahead and kawahead_n (or headkhajiit and headkhajiit_n if your replacer is set to overwrite the original khajiit files), then stick your newly renamed textures in place of the originals. The procedure is the same for all other body parts.
It's a good idea to get a DDS thumbnail viewer so you're not going at this completely blind.