There are two main ways to gain star shards: defeating Pharaoh Shadows and hunting scorpions in the afterlife. Pharaoh Shadows are worth 3 shards and scorpions are worth 2 shards. The afterlife location of Aaru (the first afterlife location unlocked) is the best way to grind out shards. I recommend unlocking the ability Favor of Osiris to replenish ammo automatically in the afterlife so that you can continue hunting scorpions. Once you have enough shards (20 for your first upgrade per crafted item, increases by 10 subsequently), go into your gear and upgrade a previously fully upgraded item, like your hidden blade or quiver.
Here is a video guide with English commentary!
Here is a playlist of guides for this game!
https://www.youtube.com/playlist?list=PL-HeW6XkKdULDFgDhklKO...