Using what ? I hear you ask ? Proportional dividers are a drawing instrument for scaling, but they make planking a wooden hull really easy.
They can be quite expensive, but I got mine at a reasonable price off fleabay. You can get real cheapies, but I've no idea if they will work satisfactorily or not. Mine were supposedly an antique, but unbranded, so they were a reasonable price.
In an ideal world, you want every plank to be tapered by about the same amount, so that you end up with a plank finishing along the length of the keel (or bulwark) and the ends all finish at the bow (or stern). You couldn't have any ends finishing off along the keel as they will be unsupported. The simple way of achieving this is to taper the planks by an amount proportional to the remaining gap. I.e. If the gap is 4 planks wide at the wide end, and 2 planks wide at the narrow end, taper the planks so that they are half a plank wide at the narrow end. Then all four planks will fit neatly.
So, how to do this. Firstly, measure the gap at the wide end. Divide the gap by the width of the planks, rounding up any part planks. In this case the gap is 24mm, and the planks are 6mm, giving us a gap of 4 planks exactly. You'd have to round it up to 5 panks if it was 25mm or more.
Set the dividers to a ratio of 4.
Place a plank in the gap and mark the positions of the frames. I usually leave it over length until I've cut the taper. It gives a bit extra wood to hold.
Set the dividers to the width of the wide end of the gap.
Now, using the other end of the dividers, making sure they don't move, use the points to mark the width on the plank at eh appropriate place. In this case, the plank is a full width, but usually it will be slightly less than the full width. Always align one point with the top of the plank.
Leave the dividers set on 4 ( or whatever number a planks you need) and set them to the gap at the narrow end.
Now mark the other end of the planks with the other end of the dividers. Again, always align one point with the top of the plank. Repeat the same process at every frame.
Now we get to play, dot-to-dot. Cut the plank to the taper you've just made. It's usually a good idea to leave it a tad oversized and then you can sand it to a perfect size, bevelling the edges as necessary.
The resulting plank looks like this. You can then glue and pin it.
Now, reset your dividers to one numebr lower, and repeat the whole process until the planking is completed. In theory each plank is identical, but in practice, they all vary slightly hence the need to remeasure each plank rather than trying to cut a batch of identical ones.
Now I just need to finish off with a photo of the completed section of planking.
You can use this method without proportional dividers, Just measure the gap with a ruler, divide by the number of planks, and mark the plank with the resultant width.