Clarifying "each other"
In Spanish, plural pronouns like nos and se often show that an action is reciprocal (done to each other). However, because these pronouns can also mean doing an action to yourselves, it can sometimes be ambiguous.
To clarify or emphasize that the action is definitely reciprocal—like the close coordination between doubles tennis partners—add the phrase el uno al otro (each other / one another) after the verb.
Using different prepositions
The standard phrase uses the preposition a (which combines with el to make al). However, if the verb you are using naturally pairs with a different preposition, you simply replace the a with that preposition.
For example, if you interfere with someone (interferir con), you use el uno con el otro. If you learn from someone (aprender de), it becomes el uno del otro (de + el).
Matching gender
Just like adjectives, the words in el uno al otro change to match the gender of the people involved. If the group performing the action is entirely female, you must use la una a la otra.
Actions with objects
Reciprocal actions often involve passing objects back and forth, or giving each other signals. Even when a direct object (like a racket or a gesture) is in the sentence, el uno al otro is still placed at the end to clarify who is receiving the action.