This should come naturally.
Easiest things to focus on are:
Combat Knives (10scrap each)
tin cans (5scrap each)
steel balls (10scrap each)
Everything uses this though,make sure that health is the first thing you craft at the start of every level and use the three bits above to use up some scrap.
Scrap is easier to come by than chemicals, especially on Ranger Hardcore, where most will come from mutated plants and fallen enemies.