We have just tipped the iceberg of Flex and Grid and it feels like there will be an intuitive nature to it but until we get there here are some things that have helped me.
I like to print cheat sheets but I found this site which has a great resource for grid and flex-box. The notes are organized by properties for the container (parent) and properties for the children with good diagrams.
Both of these have been super helpful to me. If only they would make one for CSS positioning!
If you prefer to print a cheat sheet, this is the one I use and I look at it ALL the time. It sits next to my keyboard and I add my own notes to it to make it even better. Printable Flex Cheat Sheet
The big lesson I learned on the Bakery site was to stop and think about each container (parent) to decide which is best, float, grid or flex. Just because you can make grid work was it the best choice?
I also enjoyed playing the Flex-box Froggy and Grid Garden