Instructions
Press "I" to open your inventory and equip your bucket. If you do not have a bucket in your inventory, go to your crafting bench and place iron ingots in squares one, three and five of your crafting grid. Click and drag the bucket from the crafting output box to your inventory to complete the crafting process.
Find a second water source, apart from the body of water you are attempting to flatten. Right-click on a water block to fill your bucket with water. Walk to your second body of water and place the block on one of the edges.
Return to your first water source and pick up another block of water with your bucket. Walk back to your target body of water and place the block one square away from your first block. Arrange the source blocks in a grid arrangement, alternating source blocks and the duplicate "flowing" blocks. When source blocks border flowing blocks on at least two sides, the flow stops, giving the surface a flat appearance.
Continue to fill in your grid until there are no more flowing squares in your target body of water.