Get a tape measure and measure how much cable you will need. You should walk along from your router/modem and follow your floor/ceiling/baseboard/around doors - so that you are ready to put up your cable in a way that will be unobtrusive and prevent people from tripping over it. Home Depot usually carries lengths of 100 - 500ft or more.
Secure your ethernet with cable clips (most require a hammer to install). Sample
Be sure to give yourself a little bit of slack on both ends.
Slowly work your way from one end to the other. Remember if you don’t have time to do it right the first time, when will you have time to do it right a second time?