Soakers are waterproofed woollen knitted pants that go over a modern cloth diaper such as the modern cloth diapers shown here for example:
(or even an old fashioned white square diaper if you prefer,). When you have finished knitting them you lanolise them yourself. You can get lanolin from the pharmacy, it comes in a tube and is only a few dollars. Once you have finished the soaker, turn it inside out, take a pea-sized piece of lanolin and swish it around to dissolve it in water, and then soak the soaker in there.
Make sure you use 100% wool (or close to it).
My favourite pattern is the Alice Soaker. I find it really easy to follow and it knits up really quickly, but there are loads of patterns to choose from. These ones aren't felted, they don't really need to be felted, but some are:
And longies are long pants that do the same job.
Good luck, its so much fun!