They really are more simple to do than you would think and if I remember correctly a rose was my first ever nail art design I ever did!
Actually here is a photo!
Obviously I have improved a bit since then and I have found the easiest way to do them!
Step 1: Start off with a base coat to protect your nails
Step 2: paint your nail with the colour you want as a background. For pink and red roses I like to use a minty green such as Essie 'mint candy apple' or Rimmel 'misty jade'
Step 3: with a dotting tool or the brush of your nail varnish do a circleish shape on your nail with the lightest of the two colours
Step 3: once it is dry get a thin dotting tool/end of a hair grip/cocktail stick to do a couple of curved lines inside the circle to make it look like the petals
Step 4: with the lighter of the two greens draw a short triangeish shape coming out from the rose flower
Step 5: once it is dry do a thin line with the darker green inside it
Step 6: finish with a top coat and you're done!
I've lost count of how many times I've done this design but here are a couple of designs I've done using the roses!