First of all setup your reference image by adding it to your viewport and setting it to te center of the grid as shown in the picture below.
Check Background Images then Add image, select Axis, open reference image and finally setup H and V values.
Do this for Front Ortho (1 on numpad) and Top Ortho (7 on numpad) view. 5 on numapad to switch between Ortho/Perspective view.
Creating a mesh:
Shift + A > Mesh > Cube.
Position it as shown in picture below.
- G > Z for Front View.
- G > Y for Top View.
Go to Edit mode - Tab - and press Z for wire view in both viewports.
Press CTRL + R to make a Loop Cut, position your cursor over cube to get verticall cut and Left click on it then Right click. Deselect all by hitting A then press B for Area Select, select right side of our mesh, hit X > Vertices
- Now using method above (deselect all by hitting A > B) select vertices and move them accordingly to the picture. It's important to select them by Area select because we want to select both side of the mesh.
- It's time to add more Loop Cuts (CTRL + R) and position it using the same method as above. Remember to move them only on Z Axis - G > Z or drag Z Axis (blue arrow) with Left mouse button.
- In Top view do the same to match curve from reference. This time move vertices along Y Axis.
- Go to Object Mode Tab, press Z for Solid View and add Mirror Modifier.
- Set Shading to Smooth and turn on Autosmooth.
Now you can add the rest of elements or extrude them from this base mesh. I've made some of examples in different layers in this blend file:
First layer - simple mesh with Autosmooth.
Second layer - Autosmooth with Mean Crease and SubSurf.
Third layer - Mean Crease, SubSurf with Loop Cuts.