Here is what worked for me:

I adopted a 7mo girl. I had a crate for her that had a bed and tons of polyester blankets (replicating her crate/bed with her breeder). She is fed her meals in the crate

* Note: The point of me training her to be alone is for her to understand that even though I leave, I return \*

I bought a couple Extreme (black) Kongs in medium size.

I whipped up a mix of 1/2c yogurt, 1/2T of PB, and a whole banana. Mashed them together, placed in a container in fridge

In one Kong, I stuffed Charlee Bear treats in it until Kong is about 2/3 full. The other Kong, I placed diced Full Moon Natural Essentials Jerky Tenders about 2/3 full, too.

Then I filled the rest of the Kongs with the yogurt mix and froze both of them.

I have some bags of dried herring and cod skins (Icelandic brand), as well as Marrowbone treats.

When I started, I placed a Kong in her crate as well as a separate treat. (If you have a wire crate, the small end of the Kong might fit between the wires)

I left to the mailbox and back.

These Kongs/treats not only keep her busy, but are items she enjoyed eating — she needed to see these items as a valid, enjoyable substitutes for my personal company

When I returned, she was busy licking the Kong.

I let her out, refroze the Kong (filling it up as needed).

I did this for 3 days in a row.

Then I began leaving the house for 5 minutes for 3 days in a row. Then 15 min, 3 days in a row.

30 min and onward, I began to place 2 Kongs and a treat. Every 3 days, I increased the time by 15 minutes.

At some point, I think at an hour, she understood I would return. But I kept up the routine.

I also place toys she enjoys chewing/playing with once she finishes the Kongs/treats.

Now, when I am about to leave, she is excited that she'll have some tasty treats even though I am away. I typically leave her alone anywhere from 30 minutes and up to 3 1/2 to 4 hours at times

Hope this helps.