I have always used Fabsil Gold...not just for waterproofing, as Gold also stops the fading due to the Sun's UV rays.
You can use it straight in neat, but I've also used 5ltr Fabsil to 1ltr water.
Make sure it's a good dry day to let it all take affect.
Personally I use a BIG sponge and work my way from top to bottom, like washing ure car.
But yeh, make sure bivvy/brolly is clean and dry before starting.