Here is the finished result on my 4' x 12' bed
Thank you all for the advice! Just wanted to share a picture of my raised bed all set in place. I worked the sandy base where a swingset used to be, and also tried to remove most of the landscaping fabric that was below the sand, so that my plants will be able to root deeper than the 8" height of my box. Now I have a nice mix of sand and soil as a base. I plan to line the outer borders with wet newspapers and cover it with bark mulch. I hope the weeds from the lawn don't take over in there now that I took up a lot of the landscaping fabric. Anyway, I'm excited now! I will fill with Mel's mix tomorrow or Tuesday. Yay!
P.S. The box behind the new raised bed used to be the sandbox. Last year I took that on, removing weeds and mixing it with soil. What you see coming up in there are transplanted irises, lovage, and lemon balm. This should be a nice little garden area with my new expansion!