Coming from the entrepreneur's / acquiree's perspective...
This can range from very quick and informal (up until the paperwork is filled out and taxes are filed, anyway) to quite involved.
Generally, the larger the acquisition, the more complexity you can expect since you'll have more history to vet. Expect a fair amount of due diligence in all cases, including cap table, finances, team (up to and including team interviews), product, and potentially every agreement you've ever signed. You will have to justify whatever valuation you give them, and expect some pretty harsh negotiations because it's a large amount of money at stake. What happens to everyone following the acquisition depends on why they're buying: some companies want to buy intact teams, some are more interested in acquiring the tech / IP, some want both so they can break into a market that's strategic for them. Generally, you can expect a payout of existing equity (be aware of the tax consequences of an asset purchase vs. class C merger) to make existing key players whole, coupled with some incentives to keep them working there beyond the transition period. People who aren't considered key could end up forfeiting unvested equity if they didn't sign double trigger acceleration agreements and aren't brought aboard.
It's a fairly complex topic, and I don't think there's one universal start to finish. If you're curious how to increase your likelihood of one, focus on getting traction behind a great product that another company needs (i.e. one that's a year or two ahead of the market), supported by a strong team. Those have been the two most important factors in my experience. Try to avoid excessive liabilities or negative cash flow; acquirers typically hate taking that on since it will transfer to them. Also make sure that your agreements and IP are tidy to simplify the due diligence process and increase the likelihood that the sale goes through.