I wish I had your skill
I have been drawing with computer since the first time I could use an computer... and it helps a lot to have an mother and father that is an designer. So basically I have it in my blood... but get time to sit down for a couple hours is the most difficult task.
For you I recommend to draw a lot of drawings. hand drawings, drawings on computer, etc. over time you will find thing get easy.
Personally I like to draw at High detail as possible, is it visible, then draw it! For example when I draw an ship for my AU: 1. first I start to make an box of any color with the dimension of the ship. 2. edit the shape of the box. 3. add large equipment and superstructure. 4. add all small details (pipes, windows, hatches, etc.) 5. Go over the drawing with three High visible colors for mark out shadowed area. (Light and dark green for dark shadow, and yellow for light shadow). 6. give the ship the two main color, and change all high visible colors to the correct color. (I use the piped in MSpaint to choose the color I want to change by left click the color, the I right click on the color I want to change it to, And the I choose the eraser and hold in the right mouse button and just drag the eraser over the colored area). 7. Add railings and fix template.