The process is quite involved and requires removing the display, but once you're inside, it's all pretty plain sailing.
Here's a video detailing the upgrade.
Note that even though this works now, this hack might be blocked from working by future iOS updates.
If you'd like dual physical SIMs in your iPhone but don't fancy taking it apart and making changes that are likely to affect warranty, then take a look at the adapters that SIMore has to offer. They offer both a dual-SIM and quadruple-SIM adapter for the latest iPhones. They're not as elegant a solution, but they work.
SIMore Speed X-Four 11 Pro four-SIM adapter for the iPhone 11 Pro