I love using snapdragons because they are available year round and come in a large variety of colors and shades.
I'm using pink because this design is to mimic a fresh garden look with lots of color and texture.
Follow the same rules of insertion. Straight up and down at the top and center, angling out gradually and even angling downward towards the bottom. You may have to shorten your stems a bit nearer the bottom.
I always fill in one flower variety at a time. Always count your flowers out. It's easy to get carried away and add in more flowers than you've ordered for a specific arrangement and then to fall short later.
The pinks and purples will show more contrast later when I add in the yellow of the mums and lilies.
Step back occasionally and cast a critical eye over your design to be sure it's shaping up uniformly and all around.
Don't be afraid to tuck some shorter stemmed flowers deep into the heart of the floral. This adds more depth to the look as well.