1. Go to Settings and select "Wi-Fi"
2. Tap the blue "(i)" icon next to your network
3. Scroll down & select "Manual" under "HTTP PROXY"
4. Enter your proxy's server and port
You can find these on the proxy tab.
5. Open Safari and go to "mitm.it"
If the page doesn't load, please wait a minute or two and try again (your proxy may be launching and needs a minute or two to start).
6. Select Apple
7. Tap Install
8. Enter your passcode and complete profile installation
9. Go to Settings > General > About > Certificate Trust Settings (all the way on the bottom)
10. Enable mitmproxy
If you need to take a temporary break and disconnect from your proxy, just follow up to step 3, but select "Off" instead of "Manual"
Once your proxy has terminated, it's a good idea to remove the mitm profile. Go to Settings > General > Profiles and remove the mitm entry.