Great that you have called LOCATE and have those lines marked. If you have clay soil I would not use a rotary tiller. It would make concrete if there was any moisture in the clay soils. Your PLANT beds need to be at least a foot or more higher than the lawn area so you DON'T HAVE TO take away good soils. A trench made with a flat edge shovel between the lawn and plant beds is essential.
You might have to dig down further for your lawn area and bring in a few inches of GOOD topsoil depending on your grade. Or cover with your own topsoil if it doesn't deplete your plant beds. All lawns should be graded so there is at least a 2% slope directing water where you decide WHERE the water should flow so you don't have standing puddles near your plants and grasses. Possibly into a dry well that you make by digging a hole and dumping drain rock to allow the water to filter back into the water table.
Make sure that no soil touches the bottom of your fence or siding. Ideally 4-6" below any wood. You might have to install PT dimensional lumber to make a barrier between the bottom of your fence and your fluffed up plant beds. I've built my plant beds up to 3-4' (yes feet) using turned over old sod and soils, within a few months they are reduced to 1'by decomposition of organic matter in your soil (a very good thing).
Make sure that your lawn edges are either perfectly straight with 90 degree angles or curves with continuous radiuses...When you make a curve it continues within the same radius until you go the other way and then again another continuous radius. No wiggly lines or compound radiuses if you can help it! Your eye sees the edges of the lawn far more than the homogenous green of the grass. That is why the extra effort manually digging and forming the edge and trench around your lawn is critical for a professional look.
The MOST important thing you need to do is ROLL THAT SOIL BEFORE installing sod or seed!!!! Rent a roller that you fill with water and roll your lawn, compacting the soil. Use your rake to fill in any depressions and knock down any hills. Roll again. This is the number one reason people get hilly, lumpy lawns...the soil wasn't rolled and graded properly. Honest Injun!
Make sure that you read all you can about lawns on this site, how high to cut your grass (3" no shorter for cool season grasses), sharpness of your mower blades, types of grass seed (ZERO weed seed), sun, shade (I wouldn't bother with planting a lawn in the shade, better alternatives), slope (too much won't allow water to get down into the soil profile properly), watering is critical. CRITICAL. After your lawn grasses are well rooted from sod or seed you need to train those roots to be drought tolerant. Never water everyday. Water deeply and do not water again until you see your footprints in the grass stay down and visible, then water deeply again down to 4-6" into the soil profile. Etc. Lots more to know about this creature the 'LAWN'...
Try not to get rid of good topsoil and then have to go buy new topsoil. A waste. Hope this helps fill in a few gaps...!