Note: Speaking as if this is a real protracted conflict that's probably going to last 5+ years and the Evil Empire is relatively developed. This is a vague question but in contrast to lsusr I don't actually think the details matter all that much, simply because the most effective strategies for modern empires are pretty similar.

Assuming you don't have any other black spots in your background that would prevent you from doing this, develop a legend as a suddenly highly patriotic, dutiful citizen. Spend some time researching your country's propaganda outlets and convince all of your friends and family that you have been radicalized as a card-carrying nationalist. Get in angry arguments online and in person about how justified your country is in invading Smallistan. Keep a clean nose and then join the Empire's intelligence agency, if you haven't already done so in preparation for this day. 

While you're still an analyst, make whatever briefings you are obligated to give to leadership as significantly-but-not-quite-incriminatingly misleading and counterproductive as possible. Every intelligence agency has the same problem of not supporting their counterintelligence efforts enough, because the job of a counterintelligence officer is bureaucratically analogous to "SEC compliance officer". So, without actually putting your name behind those reports in writing, privately support whatever obvious nonsense enemy double-agents are feeding your friends' reports. Then as soon as it becomes politically expedient, and after they've already done their damage, backstab your fellow officers by being the guy that discloses these problems to upper management and is credited as the brave whistleblower.

Treat Moral Mazes like an instruction manual. Ruthlessly exploit all of the opportunities your Evil Empire's bureaucracy gives you for self-interested and counterproductive behavior. Make sure you develop a reputation as an officer for the strict adherance to poorly designed and outdated rules, which your evil empire is sure to have many of. Flatter bosses to the point of disgusting coworkers, and take specific steps to make your trajectory through the ranks as demoralizing to sincere patriots as possible. Try to be the first on your teams to break any rare cultural concessions your organization has toward efficiency and away from politicking. One of the most impactful things you will be able to do during your long career is convince otherwise motivated comrades to stop trying or even quit due to the realities of bureaucratic milieu.

Once you have access to highly privileged information, wait six months or so and then begin to send anonymized messages to Smallistan. Disclose the most important operations & sources you know of at random and high intervals. Leak enough to make it clear within your organization there is definitely a spy without making it obvious that it's you. The most damaging thing you will do by leaking information like this will not be your disclosures, it will be getting the agency paranoid, making it more operationally complicated for officers to conduct "special activities" and reducing the amount of information they and their leaders are trusted with when they have to make informed decisions. Becoming a high level spy with this modus operandi without getting caught is easier than it sounds (though, to be clear, still absurdly risky), because most spies are caught via other spies. If you are able to keep your identity a secret from the nation to which you pass intelligence and don't ask them to dead drop you cash, you will be very difficult to catch, and the agency will waste ungodly amounts of organizational resources trying to "bring you to justice". 

If at some point you manage to significantly advance in rank, start slowly firing all the smartest and most uncompromised subordinates you have, and make it clear through action that you value brown-nosing and careerism above all else. Although there's no need to stop being a spy, the more power you have, the more focus you should be putting on conventionally destructive action. As an intelligence officer in an Evil Empire, there is a certain level of seniority at which you will be able to subvert and control your country's entire military strategy by just choosing which details to pass up the chain.

If Smallistan manages to lose the war anyways, parlay your nationalist and intelligence agency credentials to get a job as a member of the secret police (if that's a different branch of the government where you are). Continue your work by making sure the most important informants your country has inside the remaining insurgency are killed or ignored. Ideally however, try to convince your superiors that the insurgency doesn't actually exist, was already crushed, or at least is totally beneath the concern of your country's secret service. Maintain that lie for as long as possible. The only task force that can produce zero results, long term, is the one that was prevented from fully forming.

Eventually, coming clean about your sabotage (as opposed to being discovered) will now be the logical next step, for reasons already outlined. For maximum efficacy, bring a knife or agency-issued gun to work one day and just murder a bunch of officers over some nonsense ideology unrelated to your cause. Doesn't matter how much damage your violence actually does, just go for whatever sews as much distrust and provokes as much information siloing as possible. If that's too much for you, employ the tried-and-true closer of defecting to Rival Empire and embarassing national leadership with your daring escape and genuine confession, thereby encouraging the existing insurgency to keep on fighting through your success.

Performing well over the course of a decade, you could probably end up doing >100 million dollars of damage to the war effort; way, way more than you'd be capable of doing as a regular protestor, enemy combatant, or even domestic terrorist. A popular resistance is like a tumor in the foot of the nation; a dictator is usually willing to cut off their entire leg to be done with it. But a spy affects the ability of the nation to think and scheme, and no dictator can wipe out his KGB without removing his power.