There's many ways and many machines and tools for tube notching.
Easy way is to use drillpress and holesaw, but if you don't have them, you need to find another ways.
Actual cutting is the easiest part, specially if you have a template for drawing the lines for cutting.
Video shows my easy way how to draw a template for 90 degree tube notch.
Step 1: Cutting / Forming ...
I like to make cuts with the angle grinder using cut off wheels, the just smoothen it with the flap wheel.
I use regular pencil for drawing the lines, it doesn't disappear from the heat produced by cutting and grinding...
Step 2: Cutting / Forming...
Of course you can use other tools for forming too.
Drill / rotary tool equipped with tungsten carbide cutters, grinding stone, sanding drum..Works nicely too.
You can use even jigsaw, (sometimes you might need to shorten the blade).
For shaping ventilation ducts etc.. i have found jigsaw and reciprocating saw to be very suitable. ( no dust inside the tube, or burnt zinc coating.)
Ways are many, and now its easy task, when you have a template for it.