Question: How did Tarzan know the proper way to put the clothes on?
Answer:From the picture of his parents. He knows where everything goes, and it's not too hard to work out how to put them on.
Answer:Also, he's been thoroughly educated on human customs and society by Jane, her father, and Clayton by that point. Possibly they taught him about dressing himself during their lessons, and he continued to go about wearing the loincloth out of personal preference.
Answer: From the picture of his parents. He knows where everything goes, and it's not too hard to work out how to put them on.