Where you buy the flowers and greenery from depends on your budget and preference. You can buy them wholesale, online, or a florist. Because the arch will be kept outdoors, you need to get the timing just right: obtain the flowers the day before or the day of the wedding. Cut some greenery as well. Ferns are a classic choice, but you can use other types of greenery as well, such as eucalyptus or succulents. Fill two buckets with water, and place the flowers in one and the greenery in the other. You can also arrange the flowers based on color, shape, or size to make it easier for you to place them on the arch later. Make sure that the water is cool, and that the buckets are in a shaded area. The buckets need to be large enough to hold your flowers comfortably. If you find yourself pushing the flowers into the bucket to get them to fit, get another bucket. Make sure you keep some leaves on your flowers, to make your decoration look full and green. However, remove the leaves from the bottom 2/3 of the stem to make arranging the flowers easier. You can pull the leaves off by hand or use scissors. Oasis blocks are squares or rectangles made out of soft, green foam used in floral arrangements to keep the flowers hydrated. Soak them in water first, then secure them to the arch with cable ties. You will need a block on top of each pole, and another in the middle of the cross bar. You can buy oasis blocks in craft supply stores and floral shops. Make sure that stem is firmly wedged into the block. It needs to be deep enough so that the plant is stable, but not so deep that the stem pokes out of the other side of the block. If the stem is too long, cut it. If the stem is sticking out of the block, the plant won't get any water and will wilt. Once again, trim the stems down as needed so that they are firmly wedged in the blocks without poking out through the other side. Don't use too many flowers, however, or the arch will become too top-heavy.  Start with the largest flowers first, then fill the gaps in with the smaller ones. The ratio of flowers to greenery is up to you. A more traditional arch will have more flowers than greenery, but you can use more greenery than flowers for a more unique look. You can use more pre-soaked oasis blocks, or you can use buckets filled with water. Alternatively, you can anchor your arch with heavy, potted flowers instead of using buckets filled with sand, gravel, or rocks. How you anchor the arch with the pots depends on the arch's design:  If the arch has flat, metal plates on the bottom, set the flower pots on top of the plates to weigh them down. If the arch has stakes or narrow feet, thrust them into buckets filled with soil, then transplant the flowers.

Summary:
Buy a variety of flowers and greenery for your arch. Keep the flowers and greenery in water. Remove the lower leaves from the flowers. Use cable ties to secure pre-soaked oasis blocks to the arch. Insert your desired greenery into the blocks. Fill the gaps in with flowers. Add some flowers to the base of the arch, if desired.