You've picked out the colors for the flowers in your bouquet: pink and orange. The green stems of the flowers have no place in your bouquet, as they disrupt your color scheme. Even if you include greenery in your orange and pink bouquet, you still want to tie in the flower colors with your stems. There are several options for your stems that vary in coverage.
Cascade bouquets are different than the traditional vertically-oriented dome bouquets as the stems are either mostly or are completely hidden by the flowers. Flowers in a cascade bouquet trail down in front, creating an elegant, graceful feel. The stems are cut short, just long enough to create in appropriate hand hold. It is up to you whether you want the stems to be wrapped in ribbon or not.
The easiest way to manage this solution is if you are using pink and orange varieties of the same species of flower. If you do this, then all the stems will match in terms of thickness, length and texture, making the task easier. Pull off any leaves or rough parts of the stems so that they can be wrapped evenly. Arrange the flowers and tape (with florist tape) or band them to together. Trim the stems to the desired length and use a wide ribbon to cover the remaining stems. Start from the top of the stems, work your way down and then go back up again. Tuck the ribbon under itself to finish it at the top of the stems.
If you prefer simplicity to extravagance, consider choosing one of your two colors, either pink or orange, for the flowers in your bouquet. Then, hand tie the stems with a wide ribbon in the other color. A bow with trailing ends can create a very dramatic effect, while omitting a bow entirely can give the bouquet an understated sophistication.