Technical Details
App crashes on launch
(Assuming you've installed it with AltStore)
AltStore can “code sign” apps for a week, and they should be resigned before that time runs out. However, if you're away from your computer (AltServer) or the connection hasn't worked for some reason, the app might become unsigned and fail to launch because of that; iOS will run only apps with a valid signature.
No data has been lost, unless you've deleted the app manually. Simply open the AltStore app again on your device and verify that the resigning succeeds, and Tails will return to normal. If you can't open AltStore either, reinstall it first with your computer.
Deleting Tails will delete all data associated with it, most importantly your favorited tags. Do not delete the app as a first measure if you're trying to fix code signing issues.
How do I install AltStore Classic?
There are easy to follow instructions on AltStore's FAQ page for both Windows and Mac. AltStore doesn't currently support Linux, but it has been discussed.
Tip: Since AltStore (and other such tools) require the use of an Apple ID, I recommend creating a new one, that you only use for these tools.
Will AltStore PAL (the EU version) be supported?
No. Apple requires every app (even in “alternative app marketplaces”) to be distributed through a process called notarization, which Apple claims they use only for security purposes, but reportedly they've also blocked apps based on their features, or hindered updates due to prolonged processing times. It's basically the same crap they pull on their own App Store with the App Review process, and I don't want to waste my energy on that.
Also, notarization still requires developers to create a paid account for their developer portal, and even though the cost is small, I don't like paying that on principle. Another problem for me is that the account must be under the developer's real name or business, neither of which I want publicly associated with Tails.
I don't want to install AltStore
AltStore seems like the most frictionless solution to me, and that's the only installation method I've personally tested and verified to work, but you can always sideload the .ipa file with any sideloading tool of your choice. Just be sure to always download the .ipa file only from the official source, https://tails.safo.fi.
Be especially careful with computerless installation services that require you to install a random company's enterprise certificate or a profile onto your device. These might be “Mobile Device Management” (MDM) profiles, meant for system administrators of schools or large businesses. MDM allows access for a lot of settings and options, including clearing your device's passcode or erasing your device completely. In the past MDM profiles have also been abused to track user location, app use, email accounts, camera permissions, and browsing history.
Even if a certificate management service isn't installing an MDM profile on your device, the .ipa file they install might be a modified version of the official release. These modifications might be unnoticeable and involve only some sort of user tracking, but it wouldn't be impossible to exploit any vulnerabilities in iOS to again gain access to your entire device.1
When installed from an unofficial source, ads could also be injected into the app – if you see any ads in Tails, you're definitely using an unofficial build. By using any version of Tails with ads, you're only benefitting the party that has modified the .ipa; you're not supporting the development of this app, not the upkeep of e621, and not any artists.
Will there be an Android version?
No. Tails wasn't written as a cross-platform application. It's heavily based on iOS' UI frameworks and written entirely in Apple's Swift programming language.