Not too bad of advice, but instead of simply messaging neighbors and others conquered by the same alliance, you should organize and create an alliance yourself (I didn't feel like you implied this enough if it was your intention).
Your step 3 is awkward as well... You shouldn't have a step that requires you to inform them of other steps BEFORE the step actually occurs. But this is me being nit-picky. Just a simple order of operations error.
I'd also like to offer some additional advice on top of yours and I'll simply do so by adding to what you have with another color for my advice.
1. Get conquered. That's how it all begins. [instead of simply get conquered, you should not build units (yet) to avoid losing them when said conquer come. In addition to this, you should also be focusing on building up your structures. Focus on getting your resources to level 3 (at least) first. This way your resource income will be higher (slightly) for getting the rest off your structures.]
2. Plant agents. These agents are the long term sleeper agents. Plant a few of them in some of the enemy alliance's colonies, not all. [Planting long term agents is a good idea, but also be cautious that many larger alliances also place agents as defense against agents on important OPs. You never know for sure where enemy spies are though, just something to always be weary of.]
3. Build up friendships with any and all neighbors that are conquered from the same alliance or colony. Have them follow steps 1,4, and 5. [Building friendships is always a good idea. Diplomacy is a big aspect in this game. As I said though, I would suggest making an alliance with the one's close to you or finding good one's to make an alliance with. Always easier to defend in numbers.]
4. Plant some quicker use agents on the other colonies. Use them to launch terrorist strikes on those colonies. [This could potentially catch their attention when you don't want it. You may hurt their resources, but if caught, you will notify them of EXACTLY who they should be weary of and they will most likely come for you after.]
5. Build up troops, lots of them. [First do your research. Figure out what your conqueror is using (tanks? infantry? vehicles?). Then build against it. The more you know about your enemy, the better you can build to protect yourself. If your enemy built mostly concussive tanks, then you would be best off building explosive tanks to counter him. Also, be careful to make GOOD builds. Otherwise, you will end up getting farmed for experience. Even very experienced players end up getting farmed if they aren't paying attention, and that can consist of hundreds of units being killed with relatively low losses if you aren't careful.]
Essentially, making yourself not worth the effort to maintain you as a conquer is the best means to protect yourself. BUT, there are some (for instance, me) who will respond even more harshly against those who are annoyances by constantly streaming nukes. This is a much less likely scenario though that usually only happens if a person becomes an annoyance during peaceful times.
You may even want to try reaching out to your conqueror, or a strong member on the world for advice. There are many who are willing to help you learn (but unfortunately, there are also many who are simply rude as well). If you get a rude person, simply move on and try someone else.