For your DIY boutonniere, wait to remove the outer petals until you are designing the flowers. Those outer petals are referred to as "guard petals" and are left on at the farm to protect the inner petals from bruising and thorn tearing during the packing process.
Florists leave these petals on when they process their own flowers upon arrival. This continues to give the rose protection while drinking water and beginning to open.
As you begin your own DIY boutonnieres, remove the petals after cutting the stem to about a 1" length.
I pierce the bulb of the flower (right below the petals) with a 26 gauge wire and bend it down parallel to create a new wire stem.
This thinner stem prevents the finished boutonniere stem from getting big and bulky as you add leaves and other elements to the design.
Start floral tape up high and wrap down the stem to the bottom of the wire.
The thinner wire stem is able to be twisted into a decorative curl for a polished finish to your boutonniere later.