Welcome to Sensible Tech Tree! This is an installation guide.
You can find an album with information about the mod at the following URL:

http://imgur.com/a/W2oRQ

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Presumably you've already downloaded the zipped-up STT folder and extracted it somewhere safe.
If not, you're probably reading this on GitHub, so do that right now.
Download the folder and extract it somewhere, maybe to your desktop or the Downloads folder itself.
It's not very important, just make sure you can find it later to delete it!

The new folder you just made by extracting the download contains the following items:
 - A folder called 'Parts'
 - LICENSE.txt
 - README.txt
 - TechTree.cfg

You're going to want to figure out where KSP is installed on your device.
It's probably in the following directory (for Windows 10; if you're on anything else, good luck!):

C:\Program Files (x86)\Steam\steamapps\common\Kerbal Space Program

Once you're in the Kerbal Space Program folder, go into the GameData folder, then the Squad folder.
This is where a lot of the resources KSP needs to run are kept, including everything STT changes!
You'll notice the Squad folder contains a folder called 'Parts' too - replace it with STT's.
Now, from Squad, enter the Resources folder and replace their TechTree.cfg with STT's.
You're done!

The mod's installed, so you can delete the folder containing README.txt and LICENSE.txt if you'd like.

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Sensible Tech Tree was developed and originally distributed by Nisahu (GitHub), /u/WhereAreTheBathrooms (Reddit).