Altar decorations that you imitate don't have to have the very same colors. I'm adding pink - but it could be easily white, yellow, lavender, red or purple! Florists look at the DESIGN STYLE . . . not the colors in a photo. For even a larger difference, you could have used hydrangeas instead of spider mums.
Now we add a pink carnation to the lower left as shown.
Add one on the opposite side for balance.
At this point, just visually picture the "open spaces" between the flowers that are already in the design.
Tuck some of the flower heads in a little deeper to add depth as well as contrast. Be sure to step back occasionally and view the design from the sides as well as the front. You want a well rounded altar decorations, as these flowers are viewed from the sides by guests sitting in the pews.
Now I place a carnation in the upper right, between the two white snapdragons.
Repeat the steps, balancing out the right against the left by adding another one on the upper left. Notice I don't make it "picture perfect". That's not really necessary.
Insert the next one above the tallest spider mum, a little to the right side.
Add in a few white carnations to lighten up the dark parts of the design. Look for open spaces to insert these flowers.
Once the third white carnation is complete, we'll move on to another contrasting color. The dark blue delphinium is a linear flower just like snapdragons.
Insert the first delphinium stem in the top, angeling off the the left. Notice how this balances the pink carnation on the right.
Another delphinium is on the right, using the full length of the stalk.
You can start cutting the stems in two closer to the bottom if you wish to save on flowers, but I like using the whole stem as the idea for altar decorations is to have as big of a presence as possible.
Fill in with the last of the blue, making sure to have at least one coming straight out from the center. I know I keep harping on depth perception, but it really does set your designs apart by not having a flat look that will happen if all stems face sideways.
Now let's compare out finished design to our model below. A different container and slightly different shape, but you can see how you can pull a design pictured on the internet and make your own design following the flower placements.