Complete the game in Ranger Hardcore mode.

Scavenge everything. The main focus of the game play is to craft useful items and ammo. Although the majority of ammo can only be crafted at a workbench, you can still craft steel balls and then later on crossbow bolts from your backpack, as well as med kits and filters for the gas mask
Keep your stocks full. At every opportunity craft ammo and med kits. As said above med kits can be crafted anywhere so try to keep your quantity at maximum. Whenever you use one or two, be sure to craft more as soon as you're able. Whenever you come across a workbench, use it to refill your ammo and clean your weapons

Find suit upgrades. See the description for "Old world pictures" for a video that includes the suit upgrade locations, or rather a link to a more comprehensive guide that breaks the collectibles down so that you can just find the suit upgrades, as they're the only important collectibles for this difficulty. Many of the upgrades have uses that will make the game much more manageable, from giving you more armor, to a higher ammo count, to a radar that shows enemy locations, perfect for stealth actions. It's well worth finding some of these upgrades

Play in New Game + if possible. If you play in New Game +, you can utilize certain modifiers such as the 'my weapons' modifier that will allow you access to your previous weapons from the last full game you completed. This of course makes the combat significantly easier