Play around with this for ~3 minutes
https://codepen.io/guyroutledge/full/hgpez/
Take note of the css values on the bottom left
Skim this for ~5 minutes
https://css-tricks.com/the-css-box-model/
Don’t have to worry about understanding everything. Keep a mental index of what things exist and refer back here when you need it
Ditto with this, skim for ~5 minutes
https://scotch.io/tutorials/a-visual-guide-to-css3-flexbox-properties
Bookmark this page
http://cssreference.io/
If a css property isn’t behaving as expected, look it up on here first