How? You need to be a good gardener who already has experience in propagating seedlings. A greenhouse is important, along with the right soil and right equipment. If you have particular favourite flowers or vegetables that you like to grow, when you're propagating the seedlings fill more trays than you need and sell the extra ones. Go to your local garden centre and tell them about the seedlings you have and see whether they would be interested. You could put an ad on your front gate if you want to sell to passers by and you could also advertise in local freesheets or your newsagent's window.
How much can I earn? Anything from £100 to £2,000 a month if you work at it.