Open your inventory
Select the armor stand item (if available) or craft it
Craft it (Java/Bedrock):
Place 6 sticks in a U-shape in the crafting grid
Place 1 smooth stone in the center
Move the armor stand to your inventory
Place the armor stand in the world (use the item on the block you want)
Equip items (use the armor stand and place armor or items onto it)
Optionally adjust pose:
Use the armor stand and apply pose controls (or use tools/commands if enabled)
Optionally make it visible/invisible:
Use commands (if cheats are enabled) to set marker/no-plate properties