The first thing you do when you’re planting annual flowers is till up the soil and mix in the necessary amendments. Mix into your soil some compost and fertilizer with systemic insecticide. The systemic insecticide will keep any bugs in the soil from damaging your annuals.
Check the label on your annuals for sun requirements and suggested distance between the plants. The label will also tell you how high your plants will grow.
A rule of thumb for distance between the plants is six to 8 inches for the smaller annuals 12 to 16 inches for larger plants. This is only a general guideline.
By planting your annuals on a triangular pattern, they seem to fill in better this way and it makes for a nicer looking annual bed.