If the material isn't flat, bowedor twisted, you'll need to prop it up in order to flatten it uniformly across the full width. You will also want to have a jig to guide the 618 router as it flattens the top. If you have a good #4 or #5 hand plane, you could shave down the high spots on the inside of any bowing to help level it so it is stable on the work surface. Stick's pictures show the router running on a pair of bars.
The two pictures show variations on simple router sleds. Note that the outside long edges have an added piece on each side to restrain and guide the router as you move it across. Once the workpiece is planed or propped up, you adjust the depth of cut by raising or lowering the bit. Go lightly in small steps until a straight edge show's it is flat on one side. If you have a planer, you can level the other side, or flip it over and use the router to cut the otherside til it's flat. You will still need to do a little sanding and scraping to have a finished surface. You'll need to buy stock somewhat thicker that your desired finished thickness.
Harry's Router Sled jig uses metal bars to keep the bit cutting level all the way across, the pictured designs use some very flat Baltic Birch ply for the guide's runners. The smaller edge guides reinforce the jig and keep it flat. The opening needs to be a little wider than the biggest bit you plan to use. The opening doesn't need to be adjustable since the outside guides need to be a close fit to the router base.
Notice that the picture with the slab shows use of wedges to level off and keep the slab from rocking.
It is flattening the second side that renders the piece parallel and usable. Note that in the second picture, the maker used aluminum "L" bars.
The more I do, the less I accomplish.
Last edited by DesertRatTom; 11-29-2018 at 11:24 AM.