Most trim guys will take the arch profile to a millwork house and have the trim made on a molding shaper (planer) that can make the arched profile----Youtube videos on 'arched moldings' might give you an idea.
If you have a band saw and a router---a simple profile could be cut---but a detailed molding must be cut from the face---so a molding shaper (Planer) is needed---
New members: Adding your location to your profile helps in many ways.--M--
looks like rubber and requires lots of nails to install... cut long and fit it in as you go, you do not want to come up short with your cuts. Call a lumber supply and tell them the dimensions of the arch and the trim profile of your trim.