You can easily take simple linear measurements at ground level using a tape measure. This will provide you with the total meterage, allowing you to calculate the number of boards required and optimize the use of any off-cuts. However, measuring gable ends and bargeboards can be more challenging due to their position on the roof incline. It's advisable to seek advice from a professional when determining these measurements to ensure accuracy and safety.
Don't forget to count the number of joints and corners needed. Most joints are single-ended (300mm), but some are double-ended (500mm), which can yield two single trims if the fascia is less than 250mm. Soffits also require a soffit joint trim, which can be cut to fit the soffit width.
Additionally, measure the width and depth of the fascia and soffit boards. Remember to allow for the soffit, which may rest on the top brick course or window head, so allocate at least 25mm to 50mm for this. Also, allow about 10mm for locating in the soffit groove of the fascia.