A CAD model is made by numbers; it is essentially a "math model" of an object.
The 3D space is defined by a Cartesian coordinate system (X, Y, and Z). and elements are defined in mathematical terms i.e... A line is 2 points, a circle by a center point and radius, or three points …etc
This is not as bad as it sounds, and you don't need to be particularly good at math, the computer does most of that for you.

The model is generally viewed as a wire frame, which is a transparent skeletal view that shows ALL the lines. This can be a little disorienting at times and the fact that you see through all the objects can result in a confusing maze of lines.
The key is to use the view controls and levels to isolate the surfaces you are currently working on.
The interface allows you to open multiple views; I typically use 4 views - top front side and iso. You can place elements on 63 levels that can be selectively turned/on/off. If you need more levels, you can make multiple files and then reference them together. I typically make separate files for the engines and machine guns and then reference them in to the aircraft file.


Importing 2D Data
First step with these WW1 projects is to input the data from Corel using DXF format. Data derived this way is almost never completely accurate. First it is derived from a set of scale plans, which can contain it's own errors - then you drew it a second time, which at very least will involve a certain amount of round off and then the DXF conversion introduces some problems the most annoying of which is the fact that it turns all curved elements into line strings.
What I do is to first scale the data up to 1:1 scale and then move the different views onto separate levels rotate them into proper orientation and make sure they are all lined up with each other. In the first image above all the grey lines are imported data, the purple and green lines were constructed to generate surfaces.

Surface Generation
There are two basic types of surfaces in Microstation - simple and B-Spline
Simple surfaces are those that can be defined by any of these three methods:
1 Planar surfaces - are those that are flat and can be defined by simply drawing its perimeter and then linking the elements together.
2 Tabulated cylinder - elements that can be defined by drawing an end and then projecting the shape through space.
3 Surface of revolution - define a cross section then revolving it about centerline (see image below).

B-Spline surfaces
I'm not sure what the derivation of that term is but what it refers to is any surface that irregularly curves in two directions simultaneously. Just about any organic shape falls into this category, and is is considerably more difficult to define. B-Splines also take up a LOT of space as well as slowing down a variety of functions like screen updates and renderings. If there is ANY way to generate a surface using the tool set for simple surfaces than thats the way to go, but this is, obviously, not always possible.
The tool I use about 90% of the time to generate these types of surfaces is called movement of cross section. You define cross sections along the length of the surface (see image 1) then the software chains them together to create the surface(see image 2). It sounds simple, but it can be pretty tricky and sometimes requires a couple of attempts before you get the surface you're looking for. Another tool that I use a lot is partial delete. Here you generate a tab cylinder that defines an area that you want to remove from another surface. This is shown in images 3 and 4 below.

Here is a series of screen shots that shows the creation of a portion of the top of a Sopwith Camel. This series is a little deceptive because 95% of the work is in the creation of the cross sections themselves, once that is done the steps outlined below just take a couple of minutes :