Snip about 1/2 inch off the bottom of each stem under water. Use a very sharp knife or pruning shears. Be certain to cut the stems at an angle.

Fill your vase 3/4 full with tepid water. The warmer the water, the faster the roses will open. Colored roses open much faster than red roses, which have been treated to open slower.

Place clear tape in a grid pattern across the top of the vase, two pieces in each direction. Then go around the top of the vase with a couple of loops of the tape, to hold the ends of the grid in place. Now you have a place holder for arranging.

Pick your tallest, straightest, tightest closed rose for your first placement. It should be 1 to 1 1/2 times again as tall as your vase. This will set the height and width of your arrangement, so keep that in mind. Snip the stem at an angle, to bring the rose to the perfect height, and place it in the very center hole of your grid.

Select your next 4 roses based on tightness of head, height, and straightness of stem. Remove any large thorns that would snag the tape when you place them through the grid. Hold the roses up to the vase until the top of the head of these 4 roses reaches the bottom of the head of the first rose you placed in the vase. Snip all 4 the same length, at an angle, where they will be high enough to stand at the measured height. Place them in the 4 holes in the grid that are closest to center rose, an equal space apart.

Measure the 7 remaining roses to be about 4 to 6 inches shorter than your tallest rose, so that they will land the same distance from the bottom of the top rose and the top of the vase, cut all the same length, at an angle.

Fill in grid holes to fill in the entire vase, so that roses will be seen from every angle around the vase, once the arrangement is complete. If your tape wasn't tight, It is okay if your tape comes loose at this point, it will not be seen, and there is no need to be frustrated and worry. Each rose may not stay in the exact place you want it at this point; that, too, is okay.

Pick the prettiest, fullest, most open rose to be the center of the front of your arrangement. Even though it will be arranged "all around", you will have a front. You can even snip this rose a little shorter and put it back in, if you wish, as it will be your focal point, and should be the lowest rose toward the vase.

Take your bendable greenery and go around the outer rim of your vase, looking at it low and from a distance, to cover your mechanics around the edges, and to help you keep the bottom 7 roses where you want them. Do not get it too full at this point. They should be cut to have about 6 to 8 inches sticking out of the vase, and all leaves that would go below the waterline should be stripped.

Place 4 to 6 tall, straight, thin pieces of greenery between your top 5 roses to help them stay where you want them. Use a couple of extras anywhere a rose is not staying where you want it, to prop it by putting the stem of it in front of a rose, then pushing the top of the greenery partially behind the rose. Don't get it too full. Cut these to be about 3 inches shorter than the roses they are surrounding. They should not be all the same exact length for best look, and the tops should end in different spots.

Fill in the holes with the filler flowers. Divide your filler and make sure you fill in all around the vase pretty much equally. You don't want filler in every spot, this will make it too round and full. You want air space there, too, you just want to make sure the color is somewhat evenly distributed. Pay extra attention to your focal flower that you placed lower in the front. The filler flower will frame and accentuate your other flowers, and should always be lower and deeper in your arrangement than your main flower, the rose.

Move away from the arrangement and squint your eyes to make sure you don't see any "holes" that need a little color. Look at your arrangement from the level which it will be placed. If it is going on a 3' tall table, look at it from that view, you will see more of the top than the bottom, so your focus should be that way. If you will be seated around it, look at it from a seated position, from every angle, if it will be up high, make sure it looks good from the bottom.

Fill in with extra greenery to make sure no mechanics (tape, rose stems, etc) are showing, making sure to leave air holes.
Enjoy your arrangement! Experiment next time with different fillers, airy ferns, berries, and greenery.

Tips

Add 1 T sugar and 1 T bleach to the vase water, if you have no flower food.

Using more than one type of greenery and no filler flowers can be beautiful.

Always cut the flowers under water for longest vase life, and buy the freshest flowers you can find.
Grower's roses, or roses packed for florists, rather than retail will have "shipping petals" still on them. These are the outer petals that are left on so that the heads won't be damaged in transit and arrangement.

Before you place the roses in the arrangement, or after if you forgot, remove the one or two outer "ugly" petals by grabbing it in the center between your thumb and forefinger and wiggling back and forth from side to side to remove it from its base.

Change the water every two days, adding fresh, cool water with fresh food. On the 4th day, the ends should be cut another 1/2 inch or so and then placed back in the container. You can pull the entire arrangement out at once and turn it over to cut the bottom off, then replace it into the vase to keep from having to completely rearrange.

If you aren't going to need your arrangement for a couple of days, but have your flowers early, they should be refrigerated to make them last longer. Foods and fruits release ethylene gas, which kills flowers, so they shouldn't spend much time in a cooler with food, but would be better in a cool dark place, in cool water.

Never cut stems with a scissor. This will crimp the stem where the flower draws up water. Always use a sharp non serrated edge knife.

Cut stems on a downward angle about one inch up from the bottom, or above where stem is discolored.
Stem bottoms dry up almost instantly and will draw up air bubbles, so get your flowers into water A.S.A.P.

To force fresh flowers open, use slightly warm water. Cool water, however, is always best to preserve your flowers.