One option is to use xCode, check out the tutorial below
- Window - > Devices - > Select your device from left side list
- On the bottom, after Installed Apps list you have a little Arrow to expand logs.
- After you reproduce the issue tap on the download arrow from bottom right part of the Devices window.
Another option is to use iOS Console that you can download online.
Logs from iPhone
- Connect your phone to the computer via usb.
- Open the iOS console and see the logs appear. Before reproducing your bug "clear display" and after reproducing make sure to copy paste the log into .txt file.
NB! When you are ready to save the file, always do so in .txt format.