Metal Magic: Drilling Like a Pro 

As a aircraft engineer I would add couple of points: 1. Don't double punch with a centre punch if you can help it. 2. Put you thumb on the back of the drill and use your second finger to pull the trigger, this will make sure the force is straight down the drill, so there is a lot less chance when drill thick steel or aluminum of elongating the hole. 3. A sharp drill will reduce the amount of burr produced. 4. Reduce pressure as you get towards the far side of the item you are drilling, so you do not punch out the far side. 5. Use a piece of rubber / plastic tube or several rapes of tape over a drill bit to act as a stop if there is a chance of punching into structure behind the hole you are drilling.

The problems begin when those metal pieces have diferent forms and compound curves 😄🤷🏻‍♂️

important to have hole where you want it before drilling ...

Where can I buy scrap metal parts for practicing?

No surprise all comments are from people who "know better"

Tip angle of the drill bit is important. 118 degree is fine for wood but not metal. Buy 135 degree split points they work best.