Walking bass lines

Hi,

I'm looking to try and spice up my strumming for basic open chord songs, looking to add some bass notes before switching to the next chord. I'm just wondering how I should go about doing this? Should I focus on bass notes of a certain scale, or should I look at the notes in the actual chords i'm about to switch into? I've tried just playing around with it,and most of the time it sounds awful lol.

For example if I go from G to D, how would I go about finding a good bass note progression before strumming the D? I hope this made sense.

Thanks!