When making something with perler beads, you're going to want to have a wide range of colors, and have them organised so that you can make anything you want easily. For this you're going to want to have:

Black
Blue
Green
Yellow
Pink
Magenta
Purple
Light color (white or gray)
Orange

Step 1: Making the Outline

For this you only need the black, and you just need to follow the outside of the design.

Step 2: Making the Borders

Using the same color, black, you're going to border what you actually want to make. For this it will look like four hearts.

Step 3: Adding the Color

Now you can start to fill in the design with rainbow colors. The easiest way to do this is to find a pattern and stick with it, for me it was adding it in rows diagonally.

Step 4: Filling in Extra Spaces

For this step you're just going to want to fill in the remaining spaces with a lighter color, like gray or white.

Step 5: Ironing

After getting all of the beads placed, you are going to place a piece of wax paper on top of the beads. Plug in the iron and turn it up to a medium temperature, mine goes up to 6 and I leave it on 4. Keep ironing until it looks like the beads are connected or until it looks flat (depending on what you want it to look like).

Step 6: Final Step

Leave the wax paper on and place something on top. Place something on top so that it doesn't bend when it's cooling down. After about a minute or two you can take it off and slowly peel off the wax paper and you will have your very own design.