Learn French: Tes pas sont de plus en plus précis. - Your steps are more and more precise.

It does in negative sentences! But un pas is also a masculine noun meaning 'a step' (the negative ne... pas originally meant 'not a single step'). Here, the possessive adjective tes shows it is being used as a noun, meaning 'your steps'.

How do you use de plus en plus with an adjective?

It translates to 'more and more' and works just like a single plus (more). You place the entire phrase de plus en plus directly in front of the adjective it modifies, which is précis here.

How do I pronounce the 's' in the two plus words?

For the first plus, you make a 'z' sound (a liaison) connecting it to the vowel in en. For the second plus, because the next word (précis) starts with a consonant, its 's' is completely silent.

Shouldn't there be a plural ending on précis to match the plural tes pas?

It actually is plural! The masculine singular form of this adjective is already précis. In French, when a masculine word naturally ends in an 's' in the singular, its spelling does not change in the plural.