for the star ground, all you need is to pick a common point where it's convenient to put a large area of copper, the best place would be either the ground of the output filter or the ground side of Rsense. the idea is to keep from developing voltage drops on ground runs that would cause errors or instability in the circuit. the "common" of the star should be the ground for a circuit element that has the highest total current passing to ground. this would either be the ground for the FET (as i said, on the ground side of Rsense) ot the output filter cap ground (because of the high charge/discharge currents from the switching action of the FET. if one of these is used as the common, the voltage drop across the other ground traces will be minimized. point-to-point grounds can cause a lot of problems, especially in circuits where there are high peak currents. these currents create voltage drops along the trace. if the ground is point to point, the voltage drops interact, as some are in series with each other, especially along the long center trace in the picture. as the extremities branch in, the currents add, increasing the voltage drop along the trace. with a star ground, all of the currents sum at a single point, and the voltage drops are isolated from each other if the current along one branch increases it does not add a voltage drop to any other ground point.. think of it as a resistor network, any trace line is a resistor, with point-to point, you have several resistors in series with each other, with a star ground, no resistor is in series with another, they all tie at one common point,