Im going to go really fast since there are tons of videos on it sooooo...

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Download that tool for whatever OS you have

Get a USB, Put your XBL Account on it
Put the USB in your PC
Open up Horizon (assuming you have Horizon Downloaded)
Extract your Profile to the desktop
While Velocity is open, drag your account thats on your desktop inside Velocity
Go to the profile editor, cant tell you what exactly to click since I can't remember but it shouldn't be too hard
Mod your desired achievements
Rehash and Resign after your Done (it should be easy to find)
Once done delete the current account inside Horizon
Drag your new save that's on your desktop into Horizon
Put the USB back in your Xbox
Now put your account back on your main Hard Drive

Boom! Done, modded! congrats!

Like I said, there are tons of videos on modding your xbox live gamerscore so if this didn't help you much then I suggest you take a look at one of those, it's a simple process.