Mining is a job that can be assigned to colonists. To place the job, left click with the command tool and select the "Miner" role. Then place a miner on an appropriate spot. Appropriate spots are dark stone blocks or any of the ores amongst it. Miners must be on the ore to mine it. The arrow indicator does not mean they will mine the block it is pointing at, it is purely an aesthetic feature to allow you to control the direction your miners face while they work. Miners require a crate to work, so that they can access the stockpile.
Miners wear dark gray clothing.