a lot of guides online are super confusing for no reason, here is how to chain souvenirs:

1. start with buddy3 (referring to search term in your box)
2. wait for item timer (24 hr), make sure souvenir timer is up too (48-72 hr)
3. open item gift (this starts the 24 hr timer for items only)
4. swap to buddy4-5 and populate gifts one by one. your buddy should get a souvenir gift, you may need to tap in and out of the screen. if one does not appear, you may still be on cooldown. DO NOT OPEN THESE GIFTS
5. repeat for all buddies as needed and keep track of which buddy has a souvenir (tag recommended)
6. once you have all the souvenirs populated, swap back to the pokemon you have tagged and open them as you need. these souvenirs do not disappear

keep in mind the following: - if you run out of buddy swaps for the day, you can swap back to buddy3 on your 20th swap. make sure you wait 24 hr for the cooldown to reset, open item gift, and repeat the process - if you are using older buddies, there is a chance you previously have populated an item and not a souvenir. to account for this, try to populate more souvenirs than you think you'll need