1. Go to Grab And Go (default page label; the store can rename it) on the storefront.
2. Choose a location, then how you want to receive your order — Pickup at that location, or Delivery only if that location has "Allow Delivery" turned on for Grab & Go.
3. Pick a date the location is open for Grab & Go, then select meals and quantities.
4. Continue to Cart.

Switching to a different pickup location while you already have Grab & Go items for another location on the same date is a destructive action — the site asks you to confirm ("Each location has its own pickup/delivery. Switching locations will replace any Grab&Go items you have on the same date. Continue?").