Go into the Sparks Effect Editor tool and look for things like:
"skill_prepare_air_01_start"
"skill_prepare_air_01_loop"
"skill_cast_self_air_01_cast"
These animations are what your avatar (dummy) will do depending where they are.
For prepare effect (the attacks fx):
"RS3_FX_Skills_Air_Lightning_Prepare_Ground_02"
This will use the animation shown in the FX editor on the prepare attack.
For Cast effect it's more or less the same
"RS3_FX_Skills_Voodoo_Cast_AoE_Water_Root_01"
Adding things like ":Dummy_L_HandFX,Dummy_R_HandFX" at the end of the effect name will cause the fx to appear in the dummy's hands.
This guide may help =
https://docs.larian.game/Skill_creation