I self published earlier this year. KDP is klunky to use to say the least but also the best way to do it as it's the largest platform.
I actually had my book formatted for me via an editor using a site called Reedsy. I made a couple more tweaks after I chose the page sizes on KDP.
Then it was a case of manually sifting through it to make sure there were no pages with just a word or two on them - known as orphan words.
If you don't want to pay to have it formatted, just pick up another book and see how they've done it - starting chapter's a third of the way down the page etc..
Dickbrowne is really your man for this as he's published a few I believe.
Out of interest, what kind of book is it?