rod most prints have a sheet with the assembly drawing. so it shows all the parts and there relative position.
start simple if you have never made an engine make a few easy ones to gain experience before you dive into a large project.
Read project books explaining the details and steps of the project. The Kozo locomotive bools are great even if not building a loco it breaks down the steps shows a plan and shows in detail how to make parts that add up to a model locomotive.
I have a hard time seeing the end product
1) I encourage all in this hobby to learn free hand drawing. It trains the eye hand and mind to work together.
A copy of drawing on the right side of the brain
is a few dollars on the web.
2) I encourage all to redraw each part to be made. this will help you wrap your head around it. draw the part on a file card that can easily be placed near your tools or machine for handy reference. then file in a card file or recipe notebook when done.
many people like to redraw in 3-D cad this can really help see what the part will look like.
I think it's a must to avoid mistakes
3) I have told folks here before there are no mistakes . The home model shop is akin to a college engineering laboratory. It is a learning lab. As such expect to make a few practice pieces. Practice with a goal in mind ! Make the goal a model engine of your choice. In the beginning you may have have to make to or three practice pieces before you get one that is up to your standards . but that is OK.
4)By all means develop a plan but make it flexible fluid even.
I think the key here is what works for you but will give ideas.
a) the obvious first step is to choose a plan set.
b) choose a starting point . some will start at the base of an engine and add parts one at a time . this is a good method.
Others will start with the cylinder make the piston, heads con rod cross head etc then mount the parts on the base.
b) then assemble material and tooling for at least the first part of the project. And remember extra stock for those practice pieces.
c) choose a part and start then when complete make another part.
5) Keep a notebook white down what you do. record your successes so you can repeat them. ad record your failures so you can avoid them.
An please ask questions here folks can help with any step from selecting a plane to setting up a particular part.