Find a mangrove tree in a mangrove swamp biome
Break the mangrove propagules from the tree or leaves
Place the propagule on mud, dirt, grass, podzol, rooted dirt, or moss
Make sure the block has enough space above it
Wait for it to grow naturally over time
Use bone meal on the propagule to speed up growth
Plant multiple propagules if you want more mangrove trees
Keep the area clear so the tree has room to grow