In my opinion you'd better to use carset creator or helmet installer to install my helmets. It's much more easier.
But if you want to use wadupdater the procedure is harder.
first you have to convert tga in tex (it's why i suggest you to use carset creator or helmet installer because they do this for you). then open wadupdater.
In the top text area you have to choose the driverx_x.tex you changed, then in the lower text area you have to choose your cars.wad(i suggest you to make a backup of the cars.wad just in case.)
And then click on update wad