You have at least one Aura on the battlefield attached to a creature you control.
Yenna does not have summoning sickness.
You control at least one additional creature excluding the creature that has the Aura attached to it.
({2} magic symbol) available.
Steps
Activate Yenna by paying ({2} magic symbol) and tapping it, creating a token copy of any Aura you control attached to any other creature you control, then causing you to untap Yenna and scry 2.
Siona's second ability triggers, creating a 1/1 Human Soldier creature token.
Activate Ashnod's Altar by sacrificing the creature that has the Aura token attached to it, adding ({C} magic symbol) ({C} magic symbol) .