From a big and wide perspective, the decline is already happening, and has been for many, many decades.
It is more or less global; at the very least everywhere that is part of "modern society" is affected (not so much tribal people in the Amazon etc, although even they are affected by climate change and the infringement of modern society into their world).
It will continue for a good three or four hundred years. Look into historical examples: Rome, Maya, Chinese dynasties, Ancient Greece, loads of others.
We are heading into one of histories dark ages.
But - it will not be evenly distributed in time or geographically, so during the coming centuries there will be times and places where most people are 'doing okay' for a generation or so.
Tips:
Look after your health.
Get out of debt. Downsize your accommodation and stop buying stuff in order to get out of debt.
Don't rely on savings, pensions, investments or any kind of banking. Don't hoard banknotes and gold bars either - that is worse.
Instead, spend any spare money (after getting out of debt) on skills and tools that will make you popular with family and neighbours.
Be a good neighbour. Build your community around you.
Your future security lies in your family and community, not in any kind of 'money'.
Don't expect or assume that children should always have a better life than their parents: that is not a law of nature, it is an aberration of the last few centuries of exploitation of fossil fuels and industrialisation.