Minecraft wiki entry for Superflat_survival, associated text: By placing lava from weaponsmith shops using a bucket and pouring water over it, you can form obsidian. This way, it is possible to form a portal without ever having a single obsidian block in your inventory. However, you would need to find 5 weaponsmiths to have a bucket- so the best way may be to 'mix and match' by using some obsidian blocks and some formed from the aforementioned technique.
If you place a trail of blocks on your journey from village to village, you'll easily be able to find your way back to previously visited villages.
You can almost literally spin wheat into gold, emeralds, diamonds, and other precious items. Get all the wheat you can for trading resources.
If you have difficulty finding a resource, remember you can usually buy some from a villager. This is especially useful for acquiring a diamond sword, tool and armor, as well as redstone.
If you want to beat the game on default (where the end is inaccessible), you can gather the materials and fight the wither.