To begin you'll be wanting to download both the Blobset creator[mega.nz] and the Fixture editor[mega.nz]


If your RLL4 installation isn't on your C:, make sure you correct this in the RLL4 Location.txt found in the Blobset Creator, otherwise this'll create issues.

So upon downloading both these files unpack them into folders and go into the Fixtures editor and run the exe. Assuming you want the NRL fixtures navigate down to the "NRL Fixture Season". From there the default 2017 fixtures can be found.
The lines are as follows (team - stadiums):
0 - broncos, suncorp
1 - bulldogs, anz, belmore
2 - cowboys, willows
3 - dragons, win, winjubilee, scg
4 - eels, anz, tio
5 - knights, hunter
6 - panthers, pepper, carringtonpark
7 - rabbitohs, anz, barlowpark
8 - raiders, canberra
9 - roosters, allianz, adelaideoval, bluetongue, scg
10 - manly, brookvale
11 - sharks, remondis
12 - storm, aami
13 - wests tigers, campbelltown, leichhardt
14 - titans, cbus
15 - warriors, mtsmart, edenpark, waikato

If creating your own be sure to Save changes every round, or that progress will be lost the moment you click off it. I am probably missing a few stadiums, but I picked the core stadiums that I saw in the previous fixtures.I am also aware Eels/Tigers have Bankwest however I'm unsure whether you can tie custom stadiums to official teams with this editor.

2020 Fixtures
https://mega.nz/file/hJ43XCrZ#cO8VBAt8K2OAkVo43FgBcaIjjKzDO2K3tnbZSJZY24s

2021 Fixtures.
https://mega.nz/file/EFQmkLbR#yOIcDJplrP3NH716WovjgHoYiVNFcR575hzV8HrGm88 same case as above

2022 Fixtures
https://mega.nz/file/4dJU1TpA#VIL3sGFSGf6sKWUGp1Ucf9IjBV8euo5hbaiaGsVXONo

Once you've completed making the rounds and whatnot, make sure you've saved every round and are ready to apply it to the game.

The fixtures will save as a "careerfixtures.m3mp", which we'll be putting into the blobset files at RLL4 Blobset > RLL4Mods > m3mp and drop it in there
Once you've done that go back into the main folder of the Blobset and run the Blobset Tools.exe

Look down the list and ensure that there is a careerfixtures.m3mp If that is seen then go ahead and create the blobset.
Upon creating it you should see a completion message, and it should be complete. Obviously you'll need to create a new season to see changes.

Credit to Wouldubeinta, he's the creator of these tools and the only reason this game isn't dead entirely.

EDIT: Upon playing beyond 1 season on my own, the fixture changing can mess up other seasons, primarily with the byes being extremely lopsided, some teams have 4 and some teams will only get 1. Only advisable to play 1 season at a time with edited Fixtures, but not game breaking otherwise.