Sims learning the science skill will get 2 DNA samples of their own DNA automatically. You can clone those if you want to clone your sim. Or have a 2nd active sim with a high enough science skill get the DNA sample from your sim.
I always click on the science station to clone sims, but I think I need to have at least one other non-DNA sample in my sim's inventory for the DNA sample(s) to show up.
Also, I've been unable to clone in overstuffed households. NRaas mods don't currently remove that limitation (AFAIK).