Step 1: Recall the job of an ammeter.
An ammeter has to measure the entire current flowing through a particular branch of a circuit.
Step 2: Work out how it must be wired to do that.
The only way to make sure all of that current actually passes through the meter is to place it directly in the path of the current, that means connecting it in series, never in parallel.
Step 3: Think about what happens if it has resistance.
Adding any component in series raises the total resistance of that branch, and by Ohm's law a higher resistance would pull the current down below its true value, so the ammeter would end up reporting a smaller current than what is actually flowing.
Step 4: Fix this by choosing the resistance.
To avoid disturbing the very current it is trying to measure, the ammeter's own resistance must be brought down to essentially nothing, which is why an ideal ammeter has zero resistance.
\[ \boxed{\text{connected in series with zero resistance}} \]