Being in nature on a flat surface, you can open your menu and start a picnic. So start one so you can make sandwiches, which give you special effects.
You can use the recipe or ingredient creatively to make the sandwich, and you can choose up to six fillings and four spices. Depending on the ingredients you choose will determine what types of Meal Powers, as well as their potency.
For shiny normal type Pokémon, you will have to use creative mode and add the following ingredients. Shine, title, and encounter power for normal types, all at level 3:
Sausage x 1
Salted Herba Mystica x 2
All the normal fillings and spices can be purchased at Sure Cans, Deli Cioso, and Artisan Bakery, in most major cities.
Herba Mystica ingredients are found as rare drops from 5-star and higher Tera Raids. 6-star Tera Raids can only be accessed once you finish the main story, and they have a higher chance to drop Herba Mystica.