These tiny rodents are extremely quick and fragile and can become stressed quite easily. Some harvest mice may never become accustomed to you and so handling should be avoided unless absolutely necessary.
Harvest mice are very curious and some breeders may have animals available who are hand-tame. With these you can let them climb on your hand and up your shoulder, but be careful due to their speed and penchant for sudden hops or jumps.
With my boys I find you can pick up by cupping both hands together and encouraging the mouse to climb up. In actuality, the hardest part is actually returning them to their cage! They often find your hand more interesting than your home and tipping or rotating your hand just leads them to climb upside down. If your harvest mouse cannot be encouraged to return to his cage then you can try to very lightly lift them with your opposite hand by holding their sides and swiftly open your hand once they are safely down.
If you are confident at handling rodents then try picking your harvest mice up by the base of the tail whilst resting the body of the harvest mouse on your other hand. They may try to climb their own tail so you must move quickly, over the habitat or a large tub, and be carefully not to drop them. They can have a sharp bite.
Another option for handling Harvest mice is to place a box or container in the habitat and simply carefully pick this up once they are inside. You can use a toilet tube roll for this purpose, providing you block both ends as you lift up. Many keepers use a glass jar to help them sex their rodents, or to check overall health of their animals.