This is the same technique used by millions of blogger only.
Here what I Use ?
1.) Find Google auto suggest and related keyword and no. them according to search volume.
2.) Go to Wikipedia and search for any term related to that keyword
3.) Go to forums, quora, yahoo answer and find people’s problem related to that keyword and also get ideas from there solution.
4.)Than I go to udemy and find any course related to that keyword and get ideas from there content of table
5.) My last Keyword research tool in amazon books and there content of table
Now make a spreadsheet according to search volume and assign some LSI and Long tail keyword to some top most keyword.
It Take me one day to find write keyword.