In order to upload a custom IPA file, first find a download link for your app online and download it. Safari will download your file and will ask you which app you would like to open it with.
You should then see a button that says Open In "Sideload" If you do not see that option, then click More...and select Import with SideLoad.
Once that's done, you should then see your IPA file appear under the Files tab. Simply swipe left and hit the Sign button. From there the app will upload and be signed. You will later be prompted to download the signed version.