She sows them direct in the ground in springtime, with 1/4” soil over them, water well for a few days.
Wife loves zinnias because they grow so easily. She plants a half dozen different packets to have a constant supply of cut flowers for her desk, our dining table and to give away to others in her office. They all think she is super gardener or something. If you have plenty of sun, they grow like weeds. She has never did soil test on zennia beds, so no help there. They do survive and thrive in hot southern style sun and bloom from spring till death in the late fall.