Find a grass block
Use a tool with Silk Touch to mine the grass block
Place the grass block where you want grass to spread
Make sure the area has enough light
Keep nearby dirt blocks close to the grass block
Wait for the grass to spread naturally
Remove blocks blocking light if grass is not spreading
Use bonemeal on grass-covered areas to grow tall grass and flowers
Avoid covering the dirt with slabs, carpets, or other blocks that block spreading