The grout in my shower is cracking and crumbling.

It is best not to put new grout directly over the old. If you do, it will likely crack and fall out again very quickly because it cannot bond properly to the loose, crumbling material underneath.

To fix this properly, you should follow these steps:

1. Remove the old grout: Use a grout saw or an oscillating multi-tool with a grout removal blade to clear out the damaged areas. You generally need to remove the old grout to a depth of about 1/8 inch to give the new grout enough room to anchor itself.
2. Clean the joints: Once the old material is removed, vacuum out the dust and debris. Wipe the joints with a damp sponge or cloth to ensure they are clean and free of loose particles.
3. Check for underlying issues: If the grout is crumbling because the wall or floor behind the tile is moving or water-damaged, you may need to address those structural issues first, or the grout will simply crack again.
4. Apply new grout: Choose a grout appropriate for your tile type and spacing, press it firmly into the joints using a grout float, and wipe away the excess according to the package instructions.

Taking the time to remove the old material ensures a much longer-lasting and cleaner-looking repair.