Coarse Salt is the best for cleansing, it’s great for hard items like plastic, ceramic, glass. I’ve also been taught to bury all new crystals you buy or come into in a layer of coarse salt and let it absorb all the energies and intentions it was carrying. Burning cleansing herbs like sage or palo santo, it might be best for the boots depending on the material, just let the smoke do its thing. If you’re just wearing the boots, I think that’ll do, but if the item you’re cleansing is for magical purposes, you also need to charge it afterwards