Practice and experience.

I did not see their advice on hown to dom the patch. Doing a large patch(over 5" or so means four tape lines.

That means four lines of raised material which will mstand out if incorrectly done.

You cannot get perfectly flat, because you added tape and drywall compound.

What the pros do is to widen that added material and feather the two edges.

Example:

Drywall tape is a couple inches wide. To feather that so the result is less noticeable, you need to widen the 2" to about 12" by adding and sanding about 6" of material on each side.

Doing that takes practice. LOTS of practice generally.

The keys to success are:

Add only small amounts of mud in thin layers as smoothly as possible.

Sand as little as possible off. Sanding too much exposes the tape.

When adding mud, fill the knife and apply. Clean off the kinfe and use it to remove the excess. Clean the knife on every removal stroke. With practice, the procedure will be apply and clean with two strokes.

Use a 6" knife for the first application, a 10" for the second and a 12" for the last.

Sand parallel to the tape.

Use a good drywall primer before painting.