You read through all the advice in these blogs [1][2]. Seriously.
If you are invested in low cost index funds, most assume you can safely withdraw 4% a year and not deplete your principal. If you're retiring at 35, you might want a greater safety margin, so let's say 3%. That gives you 60K a year.
How do you retire on 60K a year? Obviously take what I'm saying with a grain of salt as I'm a relatively young person and haven't done any of this yet. But...60K is the median household income in the US, so half of families in the US live on less. If you're retired, you can probably save in ways others can't. For instance:
* Housing. You don't need to stay in a high cost of living city, so move to a much cheaper area (maybe a college town).
* Education. You have much more time, so send your kids to all public education, and use your extra time to educate them further.
* Debt. You have a ton of assets. Why hold any debt?
* Automobiles. Bike instead, if you're physically able.
* Health. Probably the hardest one since insurance in the US is tied to employment. I understand the recommended approach here is to pay out of pocket for a plan, but many have trouble with this. Of course, the standard advice is to use the extra time you have due to retirement to stay as healthy as possible, but I acknowledge this isn't a perfect plan.
[1] https://www.mrmoneymustache.com/
[2] https://www.madfientist.com/