If you buy a third party leather apple watch band, kindly check with the retailer or manufacturer if they need special care or maintain before you clean it. 

 Usually, a white genuine leather iwatch band can be cleaned per below steps:

1, Remove the white leather band from your Apple watch.

2, Use a clean dry cloth to wipe the dirt on the leather strap surface.

3, Prepare a mixture of white vinegar and warm water, one part to three, agitate the mixture until well mixed.

4, Apply little vinegar water to a clean soft cloth, clean the leather strap surface gently and completely.

5, Add mild dish soap to warm water, stir it until you see bubbles.

6, Dip new clean cloth to the soapy water, wipe the leather band to remove any stain, dirt or dust.

7, Lay the strap flat and air dry the leather band completely.

For more detail to clean your apple watch band, pls refer this blog:

https://www.veecircle.com/blogs/veecircle-blog/how-to-clean-apple-watch-band