In designing my snowflakes, I mostly just had fun with it and tried to see what geometric forms I could come up with on my own. The only one that's a complete knock-off is the one on the right in the second photo.
But if you need some (more) inspiration, check out Heodeza's nine fabulous designs (very cleverly presented on various green Pantone chips, I might add), the six designs from Bead Merrily, these 46 photos on Flickr, or just search "Perler snowflakes" or "Hama snowflakes" on Pinterest.
In a nutshell, you use a pegboard to lay out your design, and then you cover your project with the supplied ironing sheet (parchment paper is also supposed to work), and iron away. There are two types of pegboards, I noticed -- the square features even rows, where all of the pegs are lined up, whereas some of the shapes (circle and star) have the pegs lining up in alternate rows. This makes a big difference, obviously, especially when it comes to diagonals -- I found it very difficult to get a single-line diagonal row on a square board to connect when ironed because the pegs are too far apart.
The ironing is the most difficult part, in my opinion. I'd never really thought about how an iron actually works -- like, where exactly the heat comes out, but it's really important when working with fusible beads. On larger ornaments, I found that the outsides would get completely fused while the middle would be only partially melted or, worst-case scenario, not melted at all. You have to really watch out for this when you're about to turn your project over and fuse the other side -- if only some of the beads have fused together, your project will completely fall apart when you take the ironing sheet off:
The blog Holly's Hobbies suggests first ironing your ironing sheet before working on your project to get the creases out from having been folded for packaging -- this totally makes sense. I would also suggest setting your iron to low -- when I used a medium-high setting, the beads fused together too quickly and too much, and became really thin and unattractive. I found that my ornaments turned out better when I spent more time ironing them on a lower setting than trying to fuse them in 10 seconds on a higher setting.