First off find the spot you want for your pool/slide

Make the top of your pool mine is about 7 across

Now the length, it should be a bit longer then the top mine should be about 10

Now the inside start shallow then go deep

Now add water

This is what I do to add water. I make the "frame" smaller and smaller till the pool is evenly filled

There

And glowstone around the edge of the pool

Now the slide make it as tall as you want then add the part I have on top

Make it go out 3 blocks

This is a drop you will need a lot of these to make it work!

Try 1

Add a half slab then water

Doesn't drop enough

Add ladder

Fix it

Wheeeee! Ride it

Enjoy your Pool and Slide