The right sign for "in" depends on what you mean. Here I'll
show you the signs for "in" and "out" when used as basic prepositions.
However, before using this sign, ask yourself if your sentence would be fine
To do the basic sign for "in," close the fingers of your dominant hand hand,
form your non-dominant hand into a "C," and place the fingers of
your dominant hand into the "C" hand.