What I would do is get my hair wet and condition FIRST.. comb through all the tangles really well and then rinse.
Then you shampoo. I'd make sure to use something that actually can remove any of the products you've used. If you're using silicone products, you'll need something harsher. When you apply the shampoo, do so in a very careful manner so as to not tangle your hair.. work it though without messing things up too much so that it doesn't get tangled again.. rinse through carefully as well.
I've always applied my henna to damp hair, and it's always worked well!
good luck and have fun!