If you don't follow one of these options you won't be able to play. (Your name will be player524)
(best option) Download our custom launcher. The launcher will download the b2a mod which includes sound fix, skin fix, support for high res texture packs and a option to change textures to classic nuclear grass colour from classic. Download link is here: https://bit.ly/2T5dz1K
Download MultiMC at https://multimc.org/. Run the launcher and then add instance and enable old versions. Then create a new instance and use version alpha 1.2.6 and add your Minecraft account to MultiMC.
(More annoying to setup): Use the old launcher, here is a video tutorial made by epixauce (thanks)Video Link: Please join our discord if you have any trouble with the options above, our staff will be happy to help you.