To attempt this you must first have beaten The Lamb in the Dark Room.
Start a normal run as any character you're most comfortable with. I'd recommend Azazel just because flying and Brimstone can make it much quicker. You need to beat the first level boss within 1 minute of starting. If you pull it off, you'll hear Mom yell out and stomp across the floor. Go back to the starting room and there will be a large shadow of a shovel across the floor. Drop a bomb anywhere in the room, and the Broken Shovel will appear. It is an active spacebar item you must carry for the rest of the run. While holding it, Mom will constantly stomp down on you with her feet like she does in her boss fight, so you have to constantly be on the move. This can make it hard to use things like slot machines or beggars as she will usually destroy them, since her stomps count as explosions. Activating the Broken Shovel will make her quit stomping for the current room, handy for boss fights or other hectic moments. Also since the stomps are explosive damage, if you can find Pyromaniac or Host Hat, they won't hurt you anymore. You can swap the Broken Shovel for another active item temporarily, but if you leave the room without it, it will disappear.
When fighting Mom, the stomps from the Broken Shovel will stop and the fight will be as normal. Afterwards only the Negative will appear, Boss Rush will open, and the entrance to the Womb will be closed. You must then beat Boss Rush, and you must be holding the Broken Shovel at the end. If you do so, the other piece of the shovel will spawn as a reward at the end. Take it, and it will become Mom's Shovel, and the stomping will stop for good. Mom's Shovel functions identically to "We Need To Go Deeper!", letting you dig holes either to the next chapter or to crawlspaces. Grab the Negative if you haven't already, and head on to the next chapter.
Continue on as normal all the way to the Dark Room. Here, you are looking for a special room with a grave in it. It stands out fairly obviously, a weird square raised patch of dirt usually right in the middle of the room. Stand on the patch and activate Mom's Shovel. You will dig up a skeleton and a ghost will fly off the top of the screen. Don't leave the room too early or it might not unlock.