Set out a large place setting on the table as you would set it up for guests.
Measure the setting from end to end as if you were sitting at the table with that setting. Take this measurement and add 3-4 inches for seam allowance if you are making the runner yourself. You want the runner to resemble a place mat and be wider than the place setting. If you are not making it, take the place setting measurement and find a runner that is slightly wider than the place setting measurements. Most runners measure 14 to 17 inches wide.
Place the runners on the table, going across the width of the table. Set up two place settings on each runner, one on each side of the table.
Set out a centerpiece on your table if using one. Set out two large place settings directly across from each other.
Measure the bottom width of the centerpiece. This measurement represents the smallest width you have to work with for a runner. Measure the width of the table between the two large place settings. This measurement represents the largest width you have to work with. You want a runner that can accommodate your centerpiece but is out of the way of your place settings. Add about 1 and 1/2 inches to all edges to allow for seams if you are making your own.
Place the table runner in the middle of the table. It should take up about one third of the width of the table. Set out place settings around the table runner and put any centerpieces on the table runner.