Learn German: Ich lege den Stapel auf den Schreibtisch. - I am putting the pile on the desk.

You would turn the verb legen into its present participle (legend), add zu in front of it, and decline it like an adjective. Any directional phrases go before it: der auf den Schreibtisch zu legende Stapel. This construction acts as a passive obligation, meaning the pile must or is to be placed there.

Why is it auf den Schreibtisch (accusative) and not auf dem Schreibtisch (dative)?

The preposition auf is a two-way preposition. Because the verb legen (to put/lay) describes an active change of location towards a destination, you must use the accusative case (den). If the pile were already resting there, you would use the dative: Der Stapel liegt auf dem Schreibtisch.

I still sometimes mix up legen and liegen. How do I keep them straight here?

Remember that legen is transitive: you physically do it to a direct object (den Stapel), and it forces two-way prepositions into the accusative. Liegen is intransitive: the subject itself is resting, and it takes the dative. A helpful trick: legen requires action (A = Accusative), while liegen is doing diddly-squat (D = Dative).

Does der Stapel apply to any kind of pile?