The short answer
Be very cautious, and if you do it, do it small. Angel investing is appealing (you understand startups, and you want to back friends), but it doubles down on the exact risk you are already overloaded on: illiquid, high failure rate startup equity. Your own company should have first claim on your capital, your attention, and your risk budget. If you still want to angel invest, treat it as a tiny, clearly optional slice of money you can afford to lose entirely, spread across several bets rather than one, and never at the expense of your emergency fund or your family's security. For most founders, the best investment is their own company.
A curated summary to orient you, not advice. The resources below are the real value.