I think the best single bit of advice I got from the Kennedy book was to always keep in your head that it is not a one step process. You start your pattern on a good foundation, you establish your lines, fix any errors, etc., then deepen the pattern in several stepped passes to arrive at the depth you wish.
There have been several good checkering cradle designs published, that are cheap enough to make that they should be a no-brainer. The most recent published set of plans I am aware of, was in Home Shop Machinist magazine, in conjunction with an article on making a double rifle from a shotgun. It looked to be pretty much the same as the one published by the folks from Popular Mechanics (I think) many years ago, when such was done and accepted.
Checker your own Gunstock by Clyde Lammey is the article. You can see it on Google Books. I found it searching "Checkering Cradle Plans Popular Mechanics" and it was one of the first results.
Cheers
Trev