How does Macbeth meet the witches?
Macbeth meets twice with the witches; for the first time in Act 1, Scene 3, and the second in Act 4, Scene 1.
In Act 1, Scene 1, the witches confer and decide when and how to meet Macbeth; they settle upon "the heath", which is a common environment to find in Scotland, translated as a place with grass and shrubs, characterized by poor soil and somewhat marshy conditions. Sometimes translated as a "wasteland", it simply means a place that isn't especially vibrant and full of life, and it might also be a reference to the way the battlefield will look once the fighting is done. Later, when Macbeth meets them at the expected place, the meeting is characterized mostly by Macbeth reacting in curiosity and surprise, with the witches leading the conversation through their riddles and prophecies.
The second time Macbeth meets the witches is in a cave (although, if you consider Hecate to be a legitimate aspect of the play, she mentions the "pit of Acheron" as the intended meeting place at the end of Act 3, Scene 5, which might indicate either that Macbeth has actually descended into hell, or that the cave is a sort of meeting point between earth and hell). In this meeting, Macbeth is much more confident and dictating, but also anxious and indebted to the witches' favor and assistance. Here they act more as counselors and guides as Macbeth receives further prophecies, although they do nothing to actually explain their dubious meaning.
In Act One, Scene 1, the Three Witches agree to meet Macbeth following the battle "upon the heath." A heath is defined as an uncultivated piece of land with coarse grass and little vegetation. In Act One, Scene 3, the witches meet Macbeth and Banquo on a barren, desolate landscape while they are traveling to Forres, where King Duncan's castle is located. The Three Witches then proceed to give Macbeth and Banquo enigmatic prophecies before vanishing into thin air.
The second time Macbeth meets with the Three Witches takes place in Act Four, Scene 1. Macbeth meets with the Weird Sisters in a dark cavern, where the Three Witches are standing around a boiling cauldron. Macbeth immediately demands that they give him more information about his future and the witches proceed to show him more enigmatic prophecies in the form of several apparitions.