Simple stuff. If you know what you are doing.

Step 1: Research trains. Get to understand how they work, and what all those levers and flipflaps do in the cab.
Step 2: Research a train you saw and was like "ERM ME GERD LEEKIT DAT"
Step 3: Building. Here is some substeps:
1: Begin with a 5 wide (or smaller if building Narrow Gauge), 0.25 high, and a reasonable length depending on your locomotive.
2: NEVER, EVER, EVER use normal blocks. Resizeables are the key to life.
3: Walls are hard sometimes. Making a cab is the easier of walls. Usually. Keep them no more than 0.25 (unless it looks better) and don't hesitate to nudge them.
4: Engines can be made simple, such as using default car engines, or more advanced going full out to replicate the 2,000hp beasts. However, due to SP physics, I don't recommend exceeding 200hp per engine. Another thing to try is placing a propeller inside, if there is enough room.
5: Lights are so easy. Place a modded one (not in length, others are fine) where it's needed. Interior lights can be placed, but not required.
6. interiors are probably the hardest part. I don't have nudge or XML modding abilities since I'm a mobile user, but use rotators and hinges. Add small 0.25x0.25x[insert length) to the ends.

 CONTROLS TO INTERIORS CHECKLIST

1. Two brake handles. Train Brake and Locomotive Brake.
2. Power Handle / Throttle
3. Reverser (Forward/Neutral/Reverse)
4. Engine Brake - Not all locomotives feature these. Only certain diesel powered locomotives have this.

Seats and cameras are a must. Steam locomotives may have benches lining to side, and passenger cars have benches facing forwards. Some (pre 70s) had moveabke tops, for when the train redirections.

1. Train wheels- Try adding sides to the wheels, covering them. The wheels should be all wheel drive, similar to real life. Don't have more than one engine on small locomotives.
2. Small details:
   Window wipers aren't easy. Have fun trying.
   Numbers are easier for mobile users on larger builds.
   Don't attempt applying railroad names on locomotives without XML modding.

Steam Locomotive Only features:
Firebox in interior.
Smokestack
Boiler
String activated whistle (unless if making a diesel F7a or similar)

Enjoy some helpful tips. Might inspire you to go do something useful.