EDIT: 10.1.1 and 10.2 users: MobileTerminal has apparently stopped working under iOS 10 (any feedback? I'm still on 9.3.3...)!

[ 1. ] Download NewTerm (MobileTerminal) to your PC: ws.hbang.newterm_1.0~beta1_iphoneos-arm.deb

[ 2. ] Create an .ipa out of the .deb:

• Mac users can use 'iOS App signer' dantheman827.github.io/ios-app-signer (fyi, you don't need Xcode)
• Windows users can take a look here and do it like below: youtube.com/watch?v=4CEWnN_z5DI
  • Unarchive the 'ws.hbang.newterm_1.0-beta1_iphoneos-arm.deb'
  • Unarchive the resulting 'data.tar.lzma'. This will leave you with a folder named 'Applications' and a 'MobileTerminal.app' in that folder.
  • Rename the folder 'Applications' to 'Payload'.
  • Zip the folder 'Payload' and rename the file from 'Payload.zip' to 'MobileTerminal.ipa'.
  • ONLY If you are having problems creating a valid .ipa: IPAmaker can help with the last step. In that case, take a look here.

[ 3. ] Download & Install 'Cydia Impactor' from here: cydiaimpactor.com

• Sideload the 'MobileTerminal.ipa' with Impactor. (fyi, just drag the file onto Impactors application window)

[ 4. ] Download & Install 'iFunbox' from here: i-funbox.com

[ 5. ] Download the 'lib.zip' from here: mediafire.com/download/k0ad6gm28927lam/lib.zip

• Extract the containing 'lib' folder to your computer. Make sure that inside that 'lib' folder are: 'apt, cydia, dpkg, misc' (and not some 2nd 'lib' folder or sth. ...some people had that, idk why)

[ 6. ] Open iFunbox. Click on 'iBooks' (top level). Drag/Paste the 'lib' folder there. (fyi, it will land in: /var/mobile/Media/Books).

• You won't need root access (Apple File Conduit 2) in order to do this.

[ 7. ] Grab your iDevice now & Open MobileTerminal. Type it exactly how it's written!

• Type: su
• Type in your password, in order to get root access, default is: alpine (fyi, you won't see the characters you are typing)
• Type: cp -R /var/mobile/Media/Books/lib /var (fyi, be sure to note the space, mind Capitals!)

[7b.] ONLY if you are getting: 'missing var/log/apt' error:

• Type: su and password (if you closed Terminal before)
• Type: mkdir /var/log/apt

[7c.] ONLY if you are getting: 'trying to overwrite /Library/MobileSubstrate/DynamicLibraries' error:

• Type: su and password (if you closed Terminal before)
• Type: cd /Library/MobileSubstrate/DynamicLibraries (fyi, if folder doesn't seem to exist: do step 8 first!)
• Type: ls -1 (fyi, you should see 2 files now: one of them should be 'DynamicLibraries')
• Type: rm DynamicLibraries

[ 8. ] Open Cydia now. Install a simple tweak (it doesn't matter which one, but iCleaner is a good choice), in order to create the directory: /var/stash

• Type: su and password (if you closed Terminal before)
• Type: mkdir /var/stash

[8b.] ONLY if you have the 'PP Helper' in Cydia (the Chinese store, com.teiron.pphelperns):

• Uninstall it.

[ 9. ] Install 'Cydia Eraser' in Cydia.

This is optional, if things still don't work properly and/or you rather want to clean your device and start jailbreaking from anew.

[ 10. ] If you want to make sure before you use Cydia Eraser... especially if you have installed a lot of tweaks:

• Open iCleaner (install it, if you haven't), in order to see how much space you have left in: / (top line). 50MB should be enough for most scenarios. Otherwise... Cydia Eraser might fail.
• Also, If you deleted other languages with iCleaner at an earlier stage, do not, I repeat: do not use Cydia Eraser. It will fail!
• Same thing goes for manually enabled stashing (fixable tho... just revert it)
• And put your system font and screen resolution back to default, if you changed it.

[ 11. ] Uninstall 'Cydia Substrate' as a last step in Cydia, before you:

[ 12. ] Run Cydia Eraser.

[12b.] ONLY if 'Cydia Eraser' icon (or more icons) aren't showing up on your homescreens (uicache-problem):

• Ask Siri to open the app (Open Cydia Eraser)... it can be quite hard, and take multiple tries, for her to understand (you can also spell quickly: C Y D I A and then Eraser).

[ 13. ] And people, do yourself a favor and install Apple File Conduit "2" and Filza/iFile in Cydia afterwards (or at least MTerminal for christ sake...): I strongly recommend Filza, I think it works better with iOS 9!

• I also can't help but get the feeling, that file managers don't seem to rank very high amongst Millennials these days :-S... But how come, that we old stagers usually don't get ourselves into this kinda trouble (I sure as hell didn't) :D?!
• Get Filza and 13 will be your lucky number from now on :)!