Once your puppy is comfortable with being left in another part of the home to you on their own, you can begin to get them used to short periods of time alone in the house. There will be a few limitations with this training during the current lockdown, but you can still open your front door or back door and wait outside or sit in your garden if safe to do so (initially this is what we would advise you do anyway).
When you’re able to build up the time you can leave them alone, you can take the whole family on short essential shopping trips (if it's possible for everyone except you to remain in the car) and consider occasionally leaving them behind on your daily exercise if they can toilet in the garden and use up excess energy in other ways.
Prepare the area as you normally would and follow the same routine. Once your puppy is comfortable and tucking into something tasty, get yourself ready and leave the house with your keys. Return after a few minutes, before your puppy starts to become anxious. If your puppy is comfortable with this length of time, fantastic! Repeat a few times over the course of day.
Gradually increase the time you leave your puppy alone in the house to about half an hour over a period of days. Keep greetings friendly, low-key and predictable on your return, even if you find your puppy has chewed something or toileted.