Craft or obtain a diamond or netherite pickaxe
Find obsidian blocks, usually around lava and water
Make sure the lava is cooled into obsidian
Mine the obsidian with the diamond or netherite pickaxe
Wait for the block to break and drop obsidian
Collect the obsidian items
Use a pickaxe with Efficiency if you want faster mining
Use a beacon with Haste if you want even faster mining
Use a water bucket to turn lava into more obsidian if needed