Intro to this guide
This walkthrough has been written with extreme care and tested multiple times. It should show you not only how to do things, but also explain what to look out for when you do things so you don't get caught and have no idea what went wrong. If this can't help you, then nothing can really. Each step is described with as much detail as possible, without going too much over board. This text should allow you to gain silent assassin rating in every single level, on the pro difficulty. Since it should work for that, you shouldnt have a problem on any other difficulty.
Difficulty levels
You can save as much as you want, the AI is just slightly more intelligent than a post, you get the full maps with all the hints and tips form the agency and there is no evidence or notoriety. The AI is so accommodating in fact that you can easy run up at them with something not too threatening (fiberwire for example) they will look at you curious and very very slowly turn around. You can just circle them and kill them while they still think about what to do. Also they aim very badly and usually drop with one shot. This is so extreme that you can go through whole levels and kill everyone (except in The Murderer Of Crows) without too many problems. If you decide to do that though, try to silently decimate the wepaon bearing population first. Happy slaughtering.
You can save seven times per level, the AI is a bit more intelligent (normal), you get still a lot of assistance from the agency and notoriety is in effect.
Only three saves permitted in one level. The AI is more on guard than in normal. You get a reduced help from the agency. Notoriety is in effect and if there is evidence (body found, tapes etc) your rating will be affected.
The ultimate test. No saving (except between the levels for progress), the maps are pretty much blank (only your targets visible, no points of interest,...