Read widely.
Read about everything and anything and be open to seeing what you didn't expect to be there - that is how you find the best stuff.
Mark down what sticks out at you as you read.
Take notes while you read.
Wisdom not facts.
We're not looking for random pieces of information.
Your commonplace book can accumulate a mass of true wisdom - that you can turn to in times of crisis, opportunity, depression or joy.
Read and approach reading accordingly.
A commonplace book is a way to keep our learning priorities in order.
It motivates us to look for and keep only the things we can use.
After you finish a book, put it down for a week or so.
Now return to it and review all the material you've saved and transfer it to the commonplace book.
Write it down.
On index cards and label them the theme on them.
This allows categorization without source boundaries.
There are photoboxes which can be used for organization.