If you are making a product that is going to have a fermenting/ageing time at room temperature or higher of longer than 48 hours, then sanitisation/sterilisation is required and very essential. Without this step, bad bacteria, yeast, and mould may contaminate your product.
If you are fermenting for less than 48 hours and then storing the product in the fridge, for example, yoghurt, then a thorough clean with hot, soapy water will suffice. However, we always recommend starting with the cleanest equipment possible.
To sanitise and sterilise, you can use boiling water, babey bottle steriliser or iodophor steriliser. All options are fine.
If using boiling water to do this, just be aware you may need to reshape the plastic moulds, as they may soften and warp, before cooling as they dry.