What will be an easy way converting digitized negative images to positive images?

[Ofnuts]

You can use Curves with a NW-SE diagonal as a starting point and then tweak the curve:

   

(image used in example is from Wikipedia)

And if a curve works well for you you can save it as a preset.

However, if you start from a smartphone picture, the picture has likely already been over-processed by the smartphone and useful bits of info have been lost. If your smartphone can do DNG or "raw", that could be a better optikon (and the demosaicing app could have pre-cooked curves for specific film types).