Easy Lemon Buttercream Floral Sheet Cake

Prepare 2 box cake mixes per directions. 
Prepare a baking sheet around 13x17 with non-stick spray.
Preheat oven to temp on cake mix box (usually 350°)
Pour cake batter into tray filling about 3/4 full. DO NOT overfill or the cake will rise too high and may spill over. 
I had a little batter left so I made 3 cupcakes OR you can discard remaining batter.
Bake about 27-30 minutes or until toothpick comes out of the center clean.
Allow cake to fully cool and then chill before frosting. I like to make mine a day ahead if possible. Cooling will allow the cake to settle a bit.
Frost cake with buttercream being careful to spread without picking up the crumb and spreading into the frosting. Use a large knife or icing spatula to spread it as flat as possible. Frosting on a cooled cake makes this much easier!
Use a thread or floss to mark even squares. This will help guide you for where to place your florals.
Whisk together powdered sugar, milk and extract. The floral glaze should be thick but still run smooth without breaking when you lift the spoon/whisk up. When you lift the spoon it will run in a steady flow back to the bowl but it will slightly keep its shape before dissolving back in. If it is too runny, it will not hold any shape as it runs back into the bowl. Test a little bit of your glaze on a plate to make sure it's not too runny or too thick.
Mix a couple of batches of the floral frosting glaze if you're doing a full bouquet. If you're just doing one floral per square, one batch is plenty. Use food gels to color frosting in individual cups or bowls. Add glaze to piping or gallon ziplock bags and cut a very small tip off the end for fine detailing.
Pipe 3-4 stems on each square curving them so the lines at the bottom come together where a "ribbon" will be. Add florals of your choice for each stem. See tips and FAQ's below for helpful ideas!