As I always say about puppies, "Everything is a training opportunity."
I start literally the day the pup walks in the door. The time I leave the pup for is the time it takes me to walk out the front door, round the house and back in the back door. Puppy barely realises it's been left.
After a couple of days of this, (probably doing it 3 times a day,) I'll leave the pup indoors while I walk round the lawn on "Poo Patrol" so probably 4 or 5 minutes. After this for possibly 3 days, three times a day, I progress a little longer.
This time it's a walk to my corner shop. Just a quick shop and back home, no stopping to chat on the way. But after a few times of that if I meet a neighbour or friend I'll probably stop for a quick chat, so we have now progressed from 15 to 20 seconds to probably 20 minutes.
So the next step is a trip to the supermarket. Mine is only about 10 minutes from my door, so with a quick shop, 10 minute going, 10 minutes there and 10 minutes coming home, that's 30 minutes away. And so I build up. Each time a little longer than last.
Don't make a big thing about going, or coming home. It should all be as low key as possible, all "normal" and your pup will accept it as normal. I never want my pup to start to think she's been left. By 60 months old I was having medical problems and having to spend whole days in hospital. My neighbour was very good, and was popping in two or three times during the day. Not an ideal situation, but Chloe survived with no complexes! (Which probably says more about her than my training!!