Chrome bumpers don't dent easily, but when they do, most auto shops recommend replacing them. The reason is because when you dent the chrome the metal actually stretches and warps, making it nearly impossible to hammer it back into to shape. Combine that with the fact that chrome can easily crack or peel when you are trying to reshape it, and that makes replacement the only way to return the bumper to pristine condition. If you don't mind slight imperfections, you can fix a dent in a chrome bumper that will only be noticeable if studied up close and will save you the cost of expensive replacements.
Arrange yourself behind the dent. You may need to drive onto jack stands or remove the bumper. Make sure to set the emergency break so the car doesn't move during repairs.
Enlist someone to help you brace the dent from the outside to prevent it from popping out the other way. Hold a flexible silicone cutting mat over the dent.
Place the wood block over the dent's bulge. Lightly tap the block with the mallet, gradually adding more force as necessary. The wood block will diffuse the force of the blows and keep the chrome from cracking.
Check the dent from the side you are hammering. If the surface appears smooth, remove the silicone mat from the other side. Some waving in the chrome is to be expected. Repeat as necessary until the dent is as smoothed out as possible.
Polish the bumper after dent removal to help smooth the appearance of waves.
Never strike the dent from the concave side, as this will chip the chrome.