This guide is only for the new content and assumes that you've previously completed the game.

First, play until you get all your animal companions. Their help will be needed.
You will also need to complete the Mine area as it contains a needed item.

Thanks to Ellis_Is_Dead for additional info.

Find a COIN (there are several, you can get one from the payphone at S-Corp) and a MICROPHONE (behind the nightclub). Use the coin, plug in the mic and sing with the lady.

The black girl hates their singing, so give her some noise-cancelling HEADPHONES (get them from the church after helping the homeless people and the church lady).

The woman on the beach is looking for two guys. To point her to them, you'll need to give her something which you can get in the desert later.

The CODE for the lock is on the rocks on the right side of the beach.

You can get several things here. The music store has an advertisement for mail-order GUITARS, but you have no fingers with which to dial the number. The stall in the magic shop contains a WAND (crawl under the door with the mouse). Use the wand to call the number. The guitar will be delivered to your home.

Hug the chicken suit guy until he quits. Go to the road in front of the old hospital. Here he is, giving away some COSTUMES for your animals. Get three, one for each.

Two guys are camping out here. One will give you a DREAMCATCHER in exchange for a new GUITAR. You can get one in town.

The other has a relatable problem. There is some PAPER at home in the bathroom.

Give the DREAMCATCHER to the woman who’s looking for two guys. When you get back, her house of horrors will be open for business. Inside you’ll meet a hideous red crab. Give it one of those poisonous SANDWICHES from the junkyard and it’ll run away.

You might want to do the above puzzles first so that the red crab is already present, but you don’t have to. There are two small buildings here with red cables and a generator.

Dropsy is scared of the crab. Have your dog in COSTUME climb onto the generator to entice the crab. It snips the cable and gets zapped. The vent grating pops off and out climbs a baby crab. Get it to cut the antenna cable by using the bird in costume.

Moving on to the left. Use the mouse to crawl through the hole into the vent of the larger building and discover a hideous blue crab. You’ll need to incapacitate it, but luring it will require poisonous sandwiches or something that looks similar.

Dress the mouse up if you haven’t and lure the crab into the left corner, then quickly run around it to the red cable on the right (it’s tricky, try it a few times). The crab cuts the cable and gets zapped, opening the door.

The hand panel requires a GLOVE which you will find in the leftmost section of the Mine, under the obelisks. Solve the requisite puzzles to get in there. You’ve already done this if you’re a pro Dropsy player.

With the GLOVE you can finally get into the secret area. After exiting, pick up the baby crab and give it back to its parents.

That should be it for the new content!