So this is my usual method for making my own fictional ships - historical ships need a lot more looking at blueprints and the like (and Commentered is probably the best person to ask about how to do that)

I usually start by defining the waterline of the ship (the coal blocks of most of my ships), making sure the curvature is smooth and has a logical progression. This usually is determined by the length and breadth of the ship, which is usually predetermined. Periodically looking overhead at the whole water line is a good way to make sure the shape is exactly what I'm after (which will vary depending on the ship type)

Afterwards, I start blocking out the level of the main deck - taking into account if the bow section curves up, and how much it'll protrude over the water line. Things to pay attention here are the vertical line of the bow itself as well as the line nearest the widest section of the waterline. Usually this last one is the closes to a straight vertical line, as it's typically the least 'flush' part of the ship's sides.

Next step is to block in the bottom of the ship. The stern is usually the hardest as it has the most extreme curvature and must take the propellers and the rudders into account. These have to fit underneath the ship. If they protrude from an over head view, then the stern section hasn't been curved inward enough.

Once that's done, you can now fill in the vertical lines and curves of your ship, which - unless it has bulges - usually take the shape of an 'S': straightest at the section nearest the waterline, most extreme nearest the main deck level and the bottom of the ship. Once the lines are done, you can fill them in until you have a fully completed hull - if you like, here's where you can put in scuttles or portholes using stair blocks.

After you have your hull completely blocked in, then the next step is to plot where all the other pieces should go - so building a silhouette or placing placeholder circles or rough shapes will give you the approximate dimensions of everything you need to fit to make the superstructure. Keep stepping back and eyeballing whether the silhouette looks good - like if the funnels are too big, or if there's not enough space at the stern, etc.

After than, the small details are really up to how you want to render the usual paraphernalia of a ship using minecraft blocks. That's about it, really :)