Cutting the skirting boards with an offset is not difficult at all provided that you have the right tools – a mitre box and a tenon saw.
Place the skirting board upright against the sides on the inside of the mitre box (as you would place it against the wall) and clamp it tightly so that it doesn’t move while being cut. Use a tenon saw or hand saw with fine teeth and cut from left to right according to the angle that you want.
If you have a bench mitre saw on which you can run the motor along a vertical plane from left to right, lay the skirting board flat with the design side facing upwards. Cut from left to right according to the angle that you want. If the motor can only be run along a horizontal plane from left to right, position the skirting board upright and clamp it tightly so that it doesn’t move while being cut.
Whatever saw you use, always ensure that the saw blade is sharp and has fine teeth so as to minimise the risk of damage to the design of the skirting board.