I use the 3a which is better for TTL, but how it works is still the same.
Normally if you are lighting a LED etc are sourcing the current, eg volts is coming from the pin to Gnd.
With the ULN2801A/3A you are sinking the current, eg acting as a switch.
So the input side you need Gnd ( - ) and the pin .
The output side you need Gnd plus a ( + / Positive ) supply which is not necessarily the 5 volt driving the Ardunio , but any voltage up to 30 volts ?
eg the load goes from the output pin to the ( + ) supply
Don`t for get to connect both the Gnds together !! and Pin 10 is for a inductive load and should be taken to the ( + / Positive ) supply