https://pixelmonmod.com/wiki/EV_training

Simplified: If you're pokemon defeats a fast pokemon, your pokemon becomes faster. same goes with every other stat.

https://pixelmonmod.com/wiki/EV_Juices

If you set up a berry farm and make an infuser, you can quickly make items that will give 32 to a given EV.

-Caterpie - Kanto Viridian Forest (1 HP EV)

-NidoranF and Jigglypuff - Kanto Route 3 (1 or 2 HP EV)

-Slowpoke - Kanto Fuchsia City, Viridian City, Routes 6, 22, 23, and 25 (1 HP EV)

-The Chansey trainers in astrellus (2 HP EVs per chansey)

-Paldean Tauros - the spawner at "/warp farm" (2 atk EVs) (don't kill the regular tauros, they give 1 atk and 1 spe)

-Mankey - Kanto Route 22 

-Tangela - Kanto Route 21 south of pallet town (1 def EV)

-Koffing & Weezing - Kanto Pokemon Mansion (1 or 2 def EVs)

-Gastly & Haunter - Kanto Lavendar Tower (1 or 2 spa EVs)

-Tentacool & Tentacruel - Kanto Route 21 south of pallet town (1 or 2 spd EVs)

-Pidgey and Rattata - Kanto route 1 (1 Spe EV)

-Weedle - The mall spawner (1 Spe EV)