The Nomad Capitalist has a great six-part series on how to obtain citizenship by descent in different countries. If I had relatives that made me eligible for this process, I would most certainly go through it. Alas, I do not.
How to Obtain Greek Citizenship By Descent
How to Obtain Croatian Citizenship By Descent
How to Obtain German Citizenship By Descent
How to Obtain Ukrainian Citizenship By Descent
How to Obtain Spanish Citizenship By Descent
HOW TO OBTAIN FRENCH CITIZENSHIP BY DESCENT
On the same topic, Travel and Leisure has a summary article detailing the process for a few more countries. Check out:
You Can Get a Second Passport If You Have a Grandparent From One of These Countries (Video)