Well, first you need to know what chords you want. For the Keys of A you will want A (I), D(IV), E(V), and sometimes B(II). For D you will want D(I), G(IV), A(V), and sometimes E(II).
Know what 3 notes make up the Chord (major triad).
A= A, C#, E
D= D, F#, A
E= E, G#, B
B= B, D#, F#
G= G, B, D
Since you can only play two notes at once you have a lot of variations of the same chords. Common "cajun chords" include.
A= A (G string) E (D string)
C# (G string) E (D string)
E (D string) A (open A string)
A (D string) C# (A string)
C# (A string) E (open E)
C# (A string) A (E string)
See the pattern ?
D= A (G string) D (open D)
D (G string) F# (D string)
F# (D string) A (open A)
F# (D string) D (A string)
D (A string) F# (E string)
the rest you can figure out.
I have a video about Cajun seconding (which has chords in it) I use with my students when I teach Viens me Chercher (waltz in D).
heres the link to my channel, you can find the videos (3 part series) on seconding a waltz there under uploads