how do I take 300 dpi photos with my iphone 14 pro

What is there background of your question?

300dpi is an information for printing, it has nothing to do with your camera.

Your printer must be able to print in this fine.

A camera will take photos with the resolution that the camera has, or if you select a specific resolution.

Select a photo and click on Info, then you can see the resolution that this photo has.

Example:

Your camera takes photos in a resolution of 3.000 pixel on the long side.

Then a printer could give you a 10inch on the long side with 300dpi on the long side.

If you want a picture that is 20inch on the long side you could only get 150dpi, because your camera resolution is too small.

Your iPhone 14 pro has a resolution that should be sufficient for all your normal prints.

Hope this helps

Ralf