Hi, Well the best way to do this is to remove the existing lawn first. You can do this by using a sharp spade and skimming off the turf. If you have more time, you can cover the whole area with a tarp to kill everything below it and then till in the dead material and replant. I would not simply throw landscape fabric over existing turf and then plant through it. That won't work well and it will be difficult to dig through the turf layer to plant. Ideally, you should get rid of that turf to begin with. If you want, you can also simply spray the area with Round-up herbicide to kill everything and plant afterwards. The Round-up won't hurt your soil so you can plan as soon as the old plant material is dead.