First thing to do is choose a vase. I tend to prefer simple cylindrical ones. The one I used for this particular arrangement was actually from the kitchen section at HomeGoods. Pretty sure it's meant for storing flour or sugar or dog treats, but it did the trick. Depending on the vase, It can sometimes help to have a few guides to keep flowers from falling over as you start to build. A couple strips of scotch tape in a grid pattern will work! Don't get too crazy with the tape though. About a quarter to half an inch of overhang is all you need to keep the tape secure. Too much more and you might be able to see the tape after it's all set.
I usually use water that's just below room temperature. If your flowers came with food packets, definitely add that before you get started. Or, do what I did...realize halfway through adding flowers that you forgot and have to squeeze the packets into one side and swirl it around in the already full vase. I put three of the six packets of food I had into this one, but depending on the amount of flowers you can add more or less. In total, this had 24 roses, about 20 ranunculus stems, 4 hydrangea stems, and some greenery.