Do I Make Graphic Design for Print Size Correctly?

[TechnicGeek]

Hello,

I have been designing for not long time and I still feel that I don't connect some dots properly.

I am mainly interested in knowing how to make final design file print compatible.
I want to make sure it will 300 DPI and be in high resolution to be printable.

Here's my workflow:

1. I download free clipart/vector and immediately open it in GIMP
2. I do adjustments, add other elements from other files, add text
3. Before finishing and exporting graphic I set print size to 300 DPI in "Image" > "Print Size"
4. I flatten everything and export into .PNG ready to print

- Do you see any mistake in this process that would make final design not print-ready?
I am concerned about opening .JPG/.PNG graphic elements. Must they be 300 DPI before I even start working with them?

- Does it matter when I set "Print Size" to 300 DPI? Must it be done in the beginning or as long as it's done before
exporting project to final design, it does not matter?

- I often would create new page where I would modify graphic elements and I would not set DPI before I do that.
Is it bad?

Thanks!