# plants seed to his own flesh

"plants seed to satisfy his sinful nature" or "does the things he wants to do because of his sinful nature" 

# from the flesh will reap destruction

"will be destroyed by his sinful nature" 

# plants seed to the Spirit

"plants seed to please the Spirit" or "does the things God's Spirit wants"

# from the Spirit will reap eternal life

"will receive eternal life as a reward from God's Spirit"