if you put a waypoint on the loot marker on the map then it will show you exactly where the item is, including elevation, which is extremely useful for the tricky ones. if you get too close then it removes the waypoint, so you might need to add it back if you're hunting around.
Look for dark spots on the map near the item. This is usually a cave entrance. A few of them more or less require the wingsuit in order to collect.
With relics you somtimes have to look around, especially if it blends in with the walls of caves. Some of them are extremly small. Microchips you just look for a laptop in the fast travel building where you are travelling to. Lost letters is usually in a building or a cave, Just look for a dead soldier.
Thanks for the tips. Ill see again if I can find them otherwise Ill try and post approximately where I'm looking for it. I'm not too far into the game yet so I don't have a wing suit. Just got to the doctor and noticed his hang glider though.