Here we have Dora doing the come command from a distance. The easiest way to practice by yourself, is to get a long leash about 10 - 15 feet long. Let the dog get distracted and once you know they are, start backing up and say, "their name and come" FIRMLY.
As soon as they start to come, reel in the leash like a fishing reel as fast as you can while backing up. You can repeat this command and give them praise, "Good girl Come."
As soon as they come, ALWAYS have them sit first then praise.
** This is the only command that you can give a treat, so they see this command as the most important command but not every time. **