how do you attack with a spy

First, you must send your spy to an outpost, colony, or relic of your choice. Then, when you have enough infiltration ticks for the attack you want to do, click on the outpost, colony or relic. You will have a few options, but click on the icon that looks like an eye. There will be 2 panels, look at the one on the right. Scroll through it to find th attack that you want, click it, then click OK.

You will need a certain amount of infiltration ticks and energy to commence the action.

Spies do not really attack as such they can however your spy infultrates a colony relic or outpost (see above)

a spy cannot kill units (just other spies) but they do hinder them in a number of ways, for example: Locking down a target to prevent any units from leaving the target for a certain amount of ticks
Sabotage a target to reduce a type of unit's heath deplete to 1HP e.g by speading viruses, destroying fuel supplies.

Hope this helps!! Happy infultateing!!