1-5: Complete the quests Sheep Shearer and Goblin Diplomacy, which are both very easy, to get enough Crafting XP for level 5. No loss.
5-28: Kill cows in the Lumbridge cow field, and then tan into soft leather and make the best leather item for your level, all the way up to chaps. Or, just buy your leathers pre-tanned. No loss.
28+: Buy or make hard leathers and craft into hardleather bodies. This method has the lowest loss but slowest XP in F2P at only 35 XP/body. Loss is around 100 GP if not tanning and 50 GP if.
23+: Buy silver bars and smelt into tiaras. Fastest F2P XP for F2P players using silver bars at 52.5 XP/tiara. About 150 GP loss/tiara.
16+: Unstrung symbols can be made with silver bars, granting 50 XP. About 100 GP loss/symbol, or the least loss in F2P using silver. Give an extra 4 Crafting XP if strung afterwards.