Building a Good Defense

How do I go about this? What are the best units to construct? How many weapons do I need? Etc. I find the offensive military is easier to grasp.

Defense is harder to plan than offense. You only need to know one way to launch an attack. To plan a defense, you need to consider every way that you could be vulnerable.

Having a good grasp of every type of offensive strategy helps as you can then theorize what defense that you would not want to fight yourself. Obviously, most defenses start with a good command of the air. Interceptor and helicopter units are some of the most important as they respond to most attacks that occur anywhere in your country. Additional interceptor and helicopter units provided by your federation, or simply another of your countries that you fedded, is also key to have a multiple air response to each attack.

That will get you started. From there you need to consider other important defenses such as the mobile land units and look at possible garrison units. Other considerations such as ranges and your blackout periods should, also, not be overlooked. If you have experienced members of your fed, they can be an important resource in this planning and a group strategy for protection.

Good luck

Are land defense units useful? I've done some searching and it seems like it's hit and miss. There really needs to be better tutorials for the military.

Which ones? If you mean the mobile units I mentioned, units such as the mobile missile defense and the mobile air defense will respond to attacks in their vicinity and are very useful to supplement surrounding defenses.

The non-mobile land units are only useful to be used as blockers, to prevent painting, as they will only defend if attacked directly. In this role, they will be need to be dispersed in numbers around your country to be effective.

If I currently have an army entirely of professionals in the hundreds of thousands would it be prudent to only build mobile units or should I build non-mobile units mainly?

Good question. I advise and practice focusing my professionals in units that give a unique ability that a non-mobile unit cannot duplicate. Consider the utility of the mobile air and missile units I mentioned or the overall utility of mobile units that they can be quickly shuttled to any space port or depot. This can depend somewhat on your access to game cash versus gold coins. If you can mass weapons and ammo more quickly than pros, this approach will make sense.