I know this type of response is disliked by some but for the 18th century it really must be asked: With respect to whom? Because this is a huge heaping pile of "it depends"; whether we're talking about France, Britain, Sweden, Russia, pre-Fredrick the Great Prussia, post-Fredrick the Great Prussia, which reform cycle France is in, etc. etc. The difference between how a minor Germanic state and Spain would wage war would be pretty darn separate. While it's certainly possible to speak generally (and I will if that's more what you want) it would be also much simpler to approach if there was any state which specifically interested you. I'd hate to go on a huge discussion about France if you're only interested in Russia is basically what I'm saying but if you do want the glossy 'big picture' that is also an option -- I just want to clarify first.