After my daughter started escaping from her crib I knew it was time to build her a new bed. I didn't think she was quite ready for a full size yet so I thought a toddler size would work for at least the next year.
I started with the Traditional Toddler Bed plans and scaled down the size of the Rustic Headboard and incorporated that. The biggest thing I learned in the process was to measure your mattress first! I don't think I realized the size variation in crib mattresses and ended up having to recut my slats and rails to accommodate my larger mattress. Also, our mattress is 5 inches deep, which means that the safety rails only extend about an inch over the top of the bed. This seems to be fine for my daughter and she hasn't fallen out - yet - but I think if you have a very restless sleeper or just want to play it safe, you might want to add an extra inch or two to the height of the guardrails.
I also used a 2 x 6 for the foot rail as this allowed the legs to be inset further and gave it a more solid look on the bottom.