I’d hate to die like your son. Clawing at my neck, foam and bile spilling from my mouth, eyes bloodred, skin purple. Must have been horrible for you, as a Kingsguard, as a father. It was horrible enough for me. A shocking scene. Not at all what I intended. You see, I had never seen the poison work before. Tell Cersei. I want her to know it was me.
 
Much like Lady Olenna, Tell Cersei It Was Me is sweet, light, and floral on the outside.  The rest is up to you.