Effort Values make a return in Pokemon Sword & Shield, and not much has changed to system as a whole. All Pokemon can still earn a maximum of 510 Effort Values to better improve their Stats, with 252 Effort Values being the highest divisible cap to one specific Base Stat. Like previous Mainstream Pokemon Games, the main way to earn Effort Values is simply by defeating Wild Pokemon. All Wild Pokemon will give your Pokemon a certain amount of Effort Values to a specific Base Stat when defeated, the type depending solely on the Wild Pokemon's Highest Base Stat Category.
Because of this, players can usually farm Effort Values of a specific Base Stat simply by going to a certain area within the game and defeating the same Wild Pokemon over and over again. This strategy can be combined with PokeRus to earn twice as many Effort Values for each Wild Pokemon they defeat, as well as by using the Power Items of the Macho Brace (earn twice the amount of Effort Values gained), Power Weight (+8 HP Effort Values after each battle), Power Bracer (+8 Attack Effort Values after each battle), Power Belt (+8 Defense Effort Values after each battle), Power Lens (+8 Special Attack Effort Values after each battle), Power Band (+8 Special Defense Effort Values after each battle), and Power Anklet (+8 Speed Effort Values after each battle) to earn all the more Effort Values.
If you're looking to farm Speed Effort Values, head to Route 1 and defeat one Rookidee after another. For each one you defeat, your Pokemon will earn +1 Speed Effort Value. That will be +2 Speed Effort Values if your Pokemon has PokeRus, or +9 Speed Effort Values if your Pokemon is holding the Power Anklet. You can even combine PokeRus with the Power Anklet to earn all the more Speed Effort Values per Rookidee defeated!