Retaining walls are unnecessary unless you want to create special shapes or use narrower walkways. They hold the soil in place but also create hiding places for slugs. Boards, blocks, or railroad ties make good retaining walls. When you plant in your raised beds, you'll need to apply additional fertilizer because the nitrogen you added in Step 3 took care of only the 2-inch organic layer. See story on fertilizing .
Proper irrigation is important.
The soil-organic matter mixture in raised beds dries faster than clay soil. On the other hand, it is loose so it absorbs water faster. Keep walkways as dry as possible to control weeds. Place stakes at the corners of the beds to prevent hoses from damaging plants. Organic matter decomposes and disappears, so add more each year.