None of the achievements in Greenwood are difficult to get but they lack descriptions. Most of them are self explanatory and for those I will gloss over them. The ones I will focus on are the achievements that lack description and are not necessarily easy to figure out. I will assume that you know how to play the game and I will not be mentioning any puzzles or their solutions in this guide.

Simply go outside into Greenwood for the first time.

Activate the Water Tower. You cannot complete the game without doing this.

Activate the Fire Tower. You cannot complete the game without doing this.

I won't spoil anything but you will get this around the same time you learn the Earth Pentagram. It's unmissable if you plan to finish the game.

Learn all of the pentagrams. You will do this as you progress through the game. It's impossible to beat the game without learning them all so you can't miss them.

You need to activate the Earth Tower. You cannot complete the game without doing this.

Simply activate all of the Towers.

For this achievement you need to discover all of the Towers/Altars (as far as I can tell). I got the achievement when I lit the bonfire at the Altar of Light as I had already lit all of the other bonfires. I don't know if the random bonfires that aren't associated with the Towers/Altars count but I had already lit them by the time I got the achievement. You should be lighting all of the bonfires for the Collector achievement anyway. If it doesn't trigger for you just make sure you have all of the bonfires lit, even if it's day.

Acquire all of the elemental artifacts. Bear in mind you cannot acquire the elemental artifacts until the Towers are activated. Unmissable if you plan to finish the game.

This is the achievement that is the most time consuming and it is missable. In order to unlock it you need to collect all of the magic clusters. I did not do this for my initial playthrough (after achievements were implemented) and I had to do another playthrough. There are more magic clusters in the game than you need to complete the game; that's why this is missable. To make sure you don't miss it here are some tips.

-Make sure you are thoroughly searching each area. There are sometimes magic clusters tucked away in corners.
-Every time you light a bonfire (and sometimes when you free a spirit) it will generate a magic cluster.
-After you acquire the elemental crystals and activate all of the towers there will be magic clusters in the places you picked up the crystals. That is, there will be nine more magic clusters after you activate your final tower, all in places where you picked up a crystal previously so remember those places.

My achievement unlocked after I collected these nine clusters generated after the towers are activated. I had explored every area prior to this (even up to the city gates) and collected every cluster I could find. I had not yet started acquiring elemental artifacts when this achievement popped; it just happened when I picked up the last of the nine clusters that replace the element crystals. None of the clusters are hidden; you just have to thoroughly explore each area.

DO NOT ENTER THE FINAL AREA IF THE ACHIEVEMENT HAS NOT TRIGGERED! You cannot go back once you begin the final ritual so if you haven't got the achievement by that point you'll have to start the game all over again to collect magic clusters. You can speedrun the game in about 2.5 hours if you know the solutions to the puzzles and locations of all the items but it's still a pain to collect clusters all over again because you missed one or two.

Complete the final ritual and finish the game.

Hopefully this guide will help you 100% this neat little game. All of these achievements can be done in one playthrough if you pay attention and are thorough. Good luck!