There isn’t really a way, that’s the nature of pixel art. Small details will get lost, and most details will need to be redrawn or adjusted.
If the size change is considerable (such as from the examples you posted to 16x16), you’re probably better off redrawing from scratch rather than trying to resize. Those sizes are small enough that it doesn’t take long to draw the shapes you need, and the real work is in communicating the details efficiently, which isn’t something you’re going to get from scaling anyway. Scaling will also likely give you subpar shapes and poorly legible details you’ll be tempted to stick with because they “match the original”, but starting from scratch will make it easier to make decisions better suited to the small size.