First question.. what do you mean by 'selection screen'? Is that a screen defined by the SELECTION-SCREEN command? Or something else.
As far as the rest goes, it should not be that complicated. Often you have 2 structures. One of them is bound field by field to the different elements of your screen, the second has your data (either directly from the database, or from an internal table ). In the PBO you transfer the data from the 'data'-structure to the 'screen'-structure. In the PAI it goes the other way around (only in case of editable fields ofc).
So after pressing your arrow, you only have to read the next (or previous) record of the internal table in your 'data'- structure.
And why 2 structures? Because often they are not exactly alike..