The basics are you spend in game cash to get more XP so that you can level up and get the bonus donuts.
Here are the steps:
1. Turn on XP collider for 5x extra XP (optional but advisable as you'll get your donuts back*)
2. Repeatedly buy an item that has a high XP payout (see links to determine which is best for you)
3. Collect bonus donuts each time you level up
4. Sell the item to get some cash back
The people who have 10,000+ and billions of dollars spent a very long time getting this amount and building the multiplier in their town up.
Farming chart
https://forums.ea.com/en/the-simpsons-tapped-out/discussion/79249/farmingKwik-e-mart farming is definitely the best method to save cash; takes land and time to wait for the build to finish. I prefer rat traps or bloodmobiles and tend to keep a minimum cash balance (10 million) and spend any excess on farming.
note that prior to sept. 2016 the forum posts may having missing content.
*ETA: If you plan to use the KEM method and keep the collider on for 10 days, math from another
post says that in order to recoup 25 donuts for the collider your town will need to clear at least 225,000 XP per day - factors in donuts you would have earned without collider.