So what you'll need to do is: Enable Steam's console, download an older version of Lethal League Blaze (version 1.09 to be exact) and then tell Steam to not update Blaze so that your 1.09 (or PLL) files don't get erased without warning.
And, of course, you'll need to install PLL.

Disclaimer: This mod is outdated and is no longer being worked on, please do not expect for this to get updated in the near future, this is just an alternative method for playing Blaze.

Well firstly, let's list out what PLL changes so that you know what you're getting yourself into. So. . .

Instant turning.
Air turning.
LL1 Style Parry. (Grabbing a parry puts the ball back into your serve state, and also makes a funny noise letting you know you did it.)
Serves now unstick sooner.
3 Speed is added when you parry (to stop parry spamming in the corner, please refer to Minecraft Steve).
Camera is now stationary.
Option added for "Speed Modifier" (whether or not the ball is affected by diminishing returns on smashes).
Changed default settings to: 5 stocks, no powerups, 6 meter till special.
Successful parry on gives 2 meter.
Successful grab now gives full meter.
Convenience options for tech given the more flexible movement.
Hold up to superjump as Dice (as opposed to down) located in the options.

This is just a general list of the changed gameplay aspects, there's a couple bug fixes and quality of life things that I've missed here.

Firstly, you'll need to enable Steam's console. So hit control Ctrl + R and input the following command:
steam://open/console












Upon hitting "OK", you'll notice a new tab has appeared in your steam browser named "Console".

Once steam's console is open, click on the console tab (right of your profile tab), and input the following command:
download_depot 553310 553311 9050992776188261859
You'll know that it's been done right if your console window prints out this text:



Now you just wait. . .
Eventually, your console will print out another line of text saying that it's finished:

So now that you have version 1.09, you'll need to locate the download location and move all of the downloaded files into Blaze's steamapps folder.
To find the downloaded depot, go to:
[install location]\Steam\steamapps\content\app_553310\depot_553311

Then, in another file view window, you'll need to move your current version of Lethal League Blaze (located in [install location]\Steam to another folder or outright delete it.
After this, move (or preferably copy) all of the version 1.09 files into Blaze's steamapps folder.

DO NOT START THE GAME FROM HERE!
Now you will need to both disable automatic updates, and disable Steam cloud synchronization.
The unfortunate reality here is that 1.09 will not have any of your LLB unlocks, meaning you will be starting COMPLETELY OVER. Should your 1.09 version send any information to the Steam Cloud, you current version will also be wiped!

So, right click Blaze and go to properties:
















Under the "Local Files" tab, set Automatic Updates to "Only update this game when I launch it" AND (REALLY IMPORTANT) uncheck the box that says "Enable Steam Cloud synchronization for Lethal League Blaze".











Once you have done this, you should now launch Lethal League Blaze to see if the game has accepted the 1.09 version files. If the game launches, check underneath the "Options" button for your player information, the version number is the bottom line, make sure it reads "v1.09".








Once this reads "v1.09", you're done installing the base game!.

Finally, here you are! Now all you need to do is just install Project Lethal League.
Firstly you'll need to download the installer:
https://drive.google.com/file/d/1F1Rih5uw3Z9KataCkQQiofDnJzfZmTHW/view?usp=sharing

Once downloaded, open the installer and you'll be prompted with a window like this:
















Set the file location to your Blaze folder located in Steamapps ([install location]\Steam\steamapps\common\LLBlaze).
Then hit install.

Congratulations, now you can play PLL again!
You'll know that it worked because your menu background will change colors as you go through the menu:









Now I would fully recommend that you backup either base version 1.09 or back up the patched 1.09 because any updates that Team Reptile pushes out may or may not override your PLL files, meaning you'd have to start this process over again.
But besides that, go out there and have fun! If you want to play against other people you can join the following discord servers:

-=LL1 Stronghold=-
https://discord.gg/seX5a74

-=Project Lethal League=-
https://discord.gg/JV4h7Ac