You don't really need a guide for unlocking this, unlike the interrogations achievement this is pretty much unmissable as you can't fail to bring back one corpse of every alien at some point. Be warned that if you are selling corpses for money early game there comes a point where you will have a hard time finding more of certain aliens. I haven't seen a Sectoid, Floater (non-Heavy variety) or Chryssalid (my last two terror missions were all Heavy Floaters) for 3 or 4 in-game months now ...
Autopsy's are easy, just bring back a dead body, some of the rewards are more useful than others .. use the info below to prioritize your slicing and dicing. There's plenty of time in the game to complete all autopsy's so use the info below to decide which are worth doing straight away, and which can wait until a quieter moment.
Personally I found very little use for the Sectoid, Floater, Cyberdisc, Chryssalid, Berserker & Heavy Floater autopsy unlocks, but a lot depends on your play style. Also, by the time I met the Muton Elites I'd already got the Plasma Weapon Research Credit from interrogating a Muton so could have put that on the back burner until other more pressing projects were completed.
The stand out unlock is the Thin Man's Improved Medkit Project -- don't delay that one (and remember you will need 4 corpses to complete the project after the autopsy)!
Sectoid --> Manufacture: Uplink Targeting (Aim)
Sectoid Commander --> Story Progression
Floater --> Manufacture: Defense Matrix (Dodge)
Heavy Floater --> Foundry Project: Advanced Repair
Thin Man --> Foundry Project: Improved Medikit
Muton --> Foundry Project: Ammo Conservation
Chryssalid --> Manufacture: Chitin Plating
Muton Elite --> Research Credit: Plasma Weaponry (50%)
Berserker --> Manufacture: Combat Stims
Ethereal --> Manufacture: Mind Shield
Cyberdisc --> Manufacture: UFO Tracking (Boost)
Sectopod --> Foundry Project: Advanced Construction
Drone --> Foundry Project: Drone Capture