The images are very much self explanatory. For the back of the mould just make a solid wall. Mine the top part and remove the signs from top to bottom. This pretty much relies on either being lucky with finding a lava pool near you or bring a ton of buckets to a lava pool you find.
Yeah kinda, but thinking about how long it takes to get diamond and mine the obsidian it may save hours or minutes. It just depends on how fast you can find diamonds.