Have you never installed a mod before?
1. Double click on your starsector icon, look at the "mods" option, after you installed them, you will have to enable them here.
2. On your desktop, right click on the starsector icon. Mouse over "open file location", left click. In the new window, look for the "mods" folder, you will have to put the downloaded mod here.
3. On the mod page, click the "Download here" link, it will download the mod, open the mod with "winrar"(by right clicking, and choosing "Extract here"), open the "Arthrs Faction Blending + Pirate Epicness", choose ONE of the folders, for example "Arthrs Pirates But Epic", drag this ONE folder to your starsector "mods" folder.
Remember to activate the mod (see step 1).