[LG Optimus L90] AlwaysOpening:
For LG L90 D415, upgrading to Marshmallow 6.0 and doing stupid stuff along the way!

I'm new to flashing and ROMs, but I had to piece this together over eight hours last night, so I wanted to share the results in case anyone else ends up in the same situation. I'll include step-by-steps and download links below.

What Happened:

Decided to leave T-Mobile and use Hangouts over wifi for free phone service. (Which totally works, btw - I pay $15 a month for high speed wifi at home, and wifi is everywhere now, so there are just small travel gaps when I'm not reachable. You can use any wifi capable phone or tablet to do this without any kind of paid phone service.)

Got tired of T-Mobile bloatware and decided to break phone free by converting from Android 5.02 to Marshmallow.

Rooted phone with LG OneClickRoot.

Used Flashify, TWRP, CM13 and gapps to convert.

Internal storage was nearly full after conversion, so newbishly decided to delete CM13 and gapps zips from storage, deleted TWRP and Flashify folders, *and backups, doh* and then-

Tried activating Encrypt Phone function, which failed, locking me in a bootloop from LG logo to CM face to Decryption Unsuccessful - Reset Phone screens.

Being new to all this, I Googled solutions like crazy. I watched YouTube videos and tried anything I could find. If you're a newbie too and none of these work for you, now you know you're not alone.

What Didn't Work:

Powering off, then restarting while holding down Power and Volume Down. This sent it into TWRP, which would process something for a few seconds that I had no control over, and inevitably dumped me back into the bootloop.

Powering off, then restarting while holding Power, Volume Down and Volume Up. Apparently that isn't a function of this kind of phone.

Updating Drivers - Device Manager wouldn't let me install LG or universal ADB drivers. When the phone showed up at all, it registered as "MTP Device" and declared it had the correct drivers. Hand-picking drivers with Have Disk didn't work either.

All other attempts to get back into TWRP so I could wipe and reinstall.

Downloading Minimal ADB and Fastboot - "adb devices" in the command prompt couldn't find my phone.

Downloading Odin in the vain hope it would somehow see my device and magically make everything better.

Smashing phone repeatedly against forehead.

What Finally DID Work:

I found a page suggesting I power off, then immediately hold down Volume UP and Power, and plugged my phone by USB into my computer at the same time. This finally broke the cycle into a Firmware Upgrade screen.

This screen will tell you not to unplug the phone until the process is complete. Its progress bar will rest at 0% forever while you do other stuff to fix it.

While it waited, I downloaded the entire LG L90 firmware .KDZ file, and a program called LG Flash Tool 2014.

I extracted LG Flash Tool 2014, then extracted the .KDZ file into its folder.

I ran LG Flash Tool 2014, set Select Type to 3GQCT, set PhoneMode to EMERGENCY, selected my KDZ file and clicked Normal Flash.

It then asks you for country and language - select Different Country and English.

The first time it ran, all the text was ?????????, and the program errored out and closed. My phone *did* process the FirmWare Upgrade even after LG Flash Tool 2014 errored out, so I waited until it reached 100%.

I ran it again with the same settings, and this time the text was English. The Firmware Upgrade on my phone processed to 100% again. I think LG Flash Tool 2014 still errored out, but it looked successful.

On reboot, it took me to LG logo, CM face, and then a white "draw a pattern to unlock" screen, which is what I'd set as my security before I screwed everything up. However, drawing the pattern did nothing, it just stayed locked.

On re-reboot, I held Volume Down and Power, and this time it went to the Factory Reset screen. I selected Yes, and it loaded into TWRP again. PHEW.

At this point, I think my computer could see my phone again, so I plugged it in with USB and copied the CM13 and gapps zips back where they'd been.

In TWRP, I ran Wipe, then Installed CM, then gapps.

I selected Reboot System, and voila, booted properly into Marshmallow.

Unfortunately, Marshmallow was working, but Google Play was freaking out - I kept getting multiple "Unfortunately, Google has stopped working" messages every ten seconds. But I found downloading a different version of gapps, and using TWRP to wipe the system again, back it up properly, and reinstall fresh copies of CM13 and the new gapps fixed the problem.

Also, the CM13 zip I installed is apparently not the most current, so my phone told me there was a Nightly update available. I did a fresh wipe and install with that new one in TWRP. I don't know how to link to it, but your phone should tell you automatically that you can download it once Marshmallow is running.

Links to the programs I used to fix my phone and properly install Marshmallow:

LG OneClickRoot: https://drive.google.com/open?id=0BzczCljwgNnuSDZoSXBNTUptcDA&authuser=0

Flashify (you can download and install TWRP straight through this program): https://play.google.com/store/apps/details?id=com.cgollner.flashify&hl=en

CM13 (The original I used before my phone told me I could update): http://quarx2k.ru/index2.php?m&dir=lg_l90

gapps (The one that fixed the Google problem): https://www.androidfilehost.com/?fid=24052804347835438

LG Flash Tool 2014: http://lgflashtool.com/download/

LG Drivers (straight from official LG site): http://tool.lime.gdms.lge.com/dn/downloader.dev?fileKey=UW00120120425

LG D415 Firmware KDZ (To use with LG Flash Tool 2014): https://www.androidfilehost.com/?fid=24052804347759872

Theoretically, I think if you follow the steps here to root:

http://xdaforums.com/lg-l90/general/root-lg-optimus-l90-44-2-5-0-2-android-t3110257

And here to install Marshmallow:

http://www.droidviews.com/install-android-6-0-marshmallow-for-lg-l90-with-cm-13/

But use the Benzo gapps from the link above, it should install properly the first time with no Google problems.

I hope this helps, and good luck to other newbs out there! :)