Connect to internet and go onto Poké Portal from the Main Menu. Click on Tera Raids under Union Circle. From there, you can join either the Tera Raids for Walking Wake if you have Violet and Iron Leaves if you have Scarlet.
PSA: You only have one shot to get it. The Natures are fixed for that Raid Den/Day. Only way to reroll Natures is to quit your game, date skip, and restart. I got Walking Wake with a Timid Nature and a Iron Leaves with a Jolly Nature. Modest will work for Walking Wake and Adamant for Iron Leaves.
