Bring a pickaxe, torches, and food
Go to a safe mining depth
Dig a straight main tunnel
Leave 2-block-high space
Place torches regularly
Branch off side tunnels every few blocks
Mine each branch straight for several blocks
Repeat on both sides of the main tunnel
Watch for lava and caves
Collect ores as you go
Replace broken tools or bring extras
Return to the surface when inventory is full or resources run low