Sounds obvious, but choose a very sunny position for your sunflower to grow.
In winter or early spring dig over the area where your sunflower will grow and mix in lots of well rotted horse manure.
Choose a variety that is known for tall sunflowers, such as ‘Russian Giant’ or ‘Giraffe’.
Choose the largest seeds from the packet, although I often sow all of them, and select the strongest seedlings for growing in the prime position. Can give the other seedlings away or plant elsewhere in the garden.
Sow your seeds in a pot inside on a window sill or in a heated greenhouse. You can sow maybe 6-12 seeds in a 6 inch pot filled with good quality seed compost.
Don’t let the compost dry out while the seeds germinate but by the same token don’t drown the pot either or the seeds or seedlings will rot.
When the seedlings are at least 2 inches tall you can separate them or prick them out. If the weather is warm they can go straight to their permanent growing location or if it is cold still plant one on it’s own in a 6 inch pot. Again use a good quality compost.
Support. Your sunflower will fall over if you do not give it some support so you need to support it with a bamboo cane. When they are very small in a pot you can use a split cane. Use garden twine to very carefully tie your sunflower stem to the cane. Don’t pull the knot tight as you will break the stem, so make it secure but not so loose that the plant flops around. Tie the knot on the cane not on the stem of the plant. Obviously if you intend to grow a monster you may need to choose some very canes or even joing a couple securely together. You will be tying lots of new knots as the sunflower gets taller! World record attempts have even had scaffolding around them!
Slug and snails. When you plant your young sunflowers, slugs and snails will eat them overnight if you don’t protect them. I am not mad on using slug pellets but they are the best method of stopping them.
Weeding – weed away any competing weeds and make sure that every one else that does gardening in the garden knows not to weed up your sunflowers. It happened to me once! I talk from experience.
Watering and feeding. Obviously you want to grow a huge sunflower, so you need to make sure it never runs out of water, and is fed well with a good fertiliser such as Growmore or Tomorite. I’d suggest once a week, or at least once every two weeks.