What do i need to change on my DCF model if it asked to ''Ensure that the DCF properly handles purchase accounting adjustments (e.g., double-check the Cash Taxes on EBIT calculation) ?''    

The most important and widely used technique is color coding of text (e.g. blue for inputs, black for formulas).  That can, of course, be done natively using the formatting toolbar.  The nice thing about it is that in a B&W printout, the blue inputs and black font look the same, but as you work in the file, you can scan for blue ...