How do you play 'Genju of the Fields'?

Answer 1

You cast it targeting a Plains. You can pay 2 colourless mana to activate the ability, which on resolution will turn the Plains into a 2/5 creature that gains life equal to the amount of damage it deals. It's still a Plains and can tap for white mana.

If the Plains is destroyed, then you can return the Genju of the Fields card from graveyard to your hand.

There is a combat trick you can do with Genju of the Fields, and that is to activate its ability multiple times in one turn. This will 'stack' its lifegaining effect, it will gain the ability multiple times. So if you activated it three times, and it dealt 2 damage, you would gain 6 life. Spare mana can be sunk into it, to give you more life.

One important addition to the above - since M-10, the keyword 'Lifelink' is no longer stackable; multiple instances of Lifelink do not do anything extra. However, while many old lifegain cards have been updated to say they now have 'Lifelink' rather than their old wordings, Genju of the Fields' Oracle wording does not say it gives itself 'Lifelink'. It has retained its original wording, and is still a stackable effect.