It is natural to be apprehensive about where and how to position the plants. Look at the book Nature Aquarium World by Takashi Amano for artistic inspiration. Then just go ahead and do it, and remember that it won't look good until the plants grow in, so wait a month or two before repositioning anything.
Mossy plants don't like to moved too often. One rule that you should follow is to plant very densely. Remember that plants use up available nutrients and thereby prevent algae from getting a strong hold. If you try to save money by planting one plant at a time, you'll only grow an algae garden.
Excess space can be filled in with cheap fast growing plants like hornwort, which will quickly use up excess nitrates, and can be replaced later with fresh cuttings of more attractive plants. As a fast growing stem plant reaches the top of the water, you'll want to cut off the top 1/2 to 2/3 of it and replant it, leaving the rooted bottom to produce new sideshoots. In this way a small amount of a stem plant (even one cutting!) can be turned into a thick garden.
Rosette plants with roots should be pushed too far into the sand first, then pulled up so that the point where the leaves join the rootstock is above the sand. Small plants can be held down with pieces of bent wire until they root.
