This forum is as much about helping people use scripts as it is about developing them, in my experience
I'll start with the basics so please excuse me if I cover some stuff you're already familiar with. There are three types of script: Custom combat scripts (CCS), ash scripts (all end with the file type .ash), and relay ash scripts (the script name always begins with 'relay').
CCS
You can make a custom combat script very easily by going to the Adventure tab and then to the Custom Combat tab. New makes a new script, and edit makes a script editable. You can select a preexisting CCS with the drop down menu. In the case of Harvest, all you need to do with your CCS is make a new script (I suggest calling it 'meatfarming') and copy pasting the CCS from the example in the first post (those two lines are all you need to get started).
ASH
To 'install' a regular ash script you simply place it in your /kolmafia/scripts folder. Your scripts can be in any number of sub folders within this folder, for example I have a folder for all the scripts I use in aftercore: /kolmafia/scripts/aftercore. To run a script you can either type its name into the gCLI in mafia (under the graphical CLI tab) or find it in the scripts menu in mafia.
Relay
Relay scripts go in your /kolmafia/relay folder. To run a relay script you need to open the relay browser (click the map icon at the top of mafia's main window) and then find the '-run script-' drop down menu at the top of the KoL browser window that mafia opens (it's just to the left of the chat pane). Simply choose the relay script you want to run from that menu.
I think the first post provides clear enough instructions for you to go from there, but here's a quick recap. All you need to do is follow the links provided to download the scripts Harvest needs to run (these are developed by other scripters, so if you are unsure of how to set up a script other than Harvest I suggest you post your question in the script's thread, all the authors on these forums will be happy to help), and then download the three scripts attached to the first post of this thread (Harvest.ash, relay_Harvest.ash and Harvest Combat.ash) and put them in the correct folders. As I say in the first post, to set up Harvest itself and gain access to the script's settings, you just need to run the relay script (relay_Harvest.ash) from the drop down menu in the relay browser.
A small note on setting up the scripts listed under 'Requirements' in the first post: Most of these don't actually require you to do anything special to set them up (you just have to put them in the right folder so that Harvest can use them). The two exceptions to this are OCD Inventory Control.ash and eatdrink.ash (and maybe CounterChecker.ash if you have special requirements, but we'll leave that for now). Because of the amount of time it takes to set OCD up for the first time, Harvest comes preloaded with all of the data OCD needs to run (assuming you're farming the castle). You should aim to properly set OCD when you get a chance, but it'll work quite happily as is (just make sure you run relay_OCD_db_Manager.ash once before running Harvest for the first time). You should have no trouble setting eatdrink up as long as you follow the instructions in the first post of that thread (linked in the first post) carefully.
If you're still unsure of anything (or everything) post here again saying so and I'll do my best to clarify
EDIT: Once you've got Harvest working and you're configuring your settings, if you're not sure what a setting does you can simply mouse over it for a description.
