The easiest (ie very few calculations) is to prepare 1M KH2PO4 (acid form) and 1M K2HPO4 (basic form) solutions and take a defined volume of either of the two and adjust pH with the other solution. As the wanted pH is roughly 1 pH unit beyond the second pKa of phosphoric acid then you would need 10 times more of K2HPO4 than KH2PO4. So by mixing 100mL of the former with 10mL of the latter you shouldn't be too far from 7.8...
The easiest (ie very few calculations) is to prepare 1M KH2PO4 (acid form) and 1M K2HPO4 (basic form) solutions and take a defined volume of either of the two and adjust pH with the other solution. As the wanted pH is roughly 1 pH unit beyond the second pKa of phosphoric acid then you would need 10 times more of K2HPO4 than KH2PO4. So by mixing 100mL of the former with 10mL of the latter you shouldn't be too far from 7.8...