A couple of keys to growing moonflowers successfully:
If you live in a colder climate start seeds early. Plant indoors, under lights, approximately four weeks before night time temperatures are reliably greater than 50 °F.
Soak seeds in warm water for four to eight hours before you plant them.
Start seeds in plantable pots (peat or Cow Pots work well), because the roots don't like to be disturbed. That said, I have bought moonflower seedlings in plastic pots and have transplanted them successfully.
Once night time temperatures have reached 50°F, plant seedlings in a large pot.
If you live in a warmer climate you can sow your seeds directly into a pot
You want soil that is well drained, but doesn't have too much fertilizer. If there is too much fertilizer, you will get lots of leaves, but not many flowers.
Place your container in full sun.
Keep soil moist.