the groove for your t molding should be directly in the middle of your wood, so what i'd reccomend is starting in the midle and going to one end and then flipping it over and going to the other corner.
Unless you are 100% positive you are DEAD NUTS CENTERED,
do not do this.
Jabba has posted the correct way to do this, and as a nice added side benefit, using his method will remove any possibility of tearout at the corners. While the chance of tearout is slim, it could happen, rendering your entire panel useless or requiring an extensive patching job you prolly won't be happy with.
Go get a Quick-Grip clamp and use some of your scrap wood to line up and clamp it to the edge. Then, as Shape D recommends, start in the middle of your SCRAP piece of wood and work to the other end, which should also have a piece of scrap clamped to it.