The holes in a homemade olla made from clay pots can be small. It can be hard to fill them. I have found it easiest to use a funnel with a tip small enough to fit in the hole. As shown in the picture above.
Funnels are cheap and work quite well. Especially if you do not make your ollas with PVC pipe filling extensions. By not adding an extension you can keep the cost per olla down. However, PVC extensions do have plenty of great benefits though.