The first thing you'll want to do is go to Spider Hell open the door behind spawn. To do this you will need to squish 250 spiders.

Once you open the door you'll notice a wall with two torches. I don't remember if they start lit, but if so then extinguish them either by pressing the use key on them or using your power washer. You will also notice two teleporters, one on each side of the entrance. Take the one on the left.

You'll teleport in front of a large stone box. Go behind it to find a blue flame in a brazier. Next to the brazier are a couple of torches, take one and light it with the blue fire you need the blue flame to open every secret door in the level. After it's lit go back from where you came from and take the teleporter back. Light the two torches on the wall with the blue flame and the wall will open.

This next part is very important that you follow in order and do not deviate. If you take any portal except the ones I describe then you won't get teleported to where you need to go. Assuming this isn't unique to each playthrough you need to go directly between these levels without going into any others or you will have to start all over.

1. Spider Hell
   
2. HEM
   
3. Artois Manor
   
4. Rancho Sauardo
   
5. Downtown Arachnitopia
   
6. Spider Hell

In order to do this follow this order exactly.

1. In Spider Hell go through the door I mentioned above and take the portal to the back right, this will lead you to HEM.
   
2. Head to the model home and enter the portal in the closet. This will lead you to Artois Manor.
   
   1. Go straight through the door ahead of you.
      
   2. Immediately go left after that door and follow that hallway until you reach another door.
      
   3. Go through this next door and immediately go right and take the door to the right immediately around the corner.
      
   4. Go through the next door and turn left, you should see a door behind an armchair, to the left of a fireplace, and below a ceiling vent. A green glow emanates from it. Go through the door and enter the portal.
      
3. Head to the kitchen and hop on the dining table, the portal is above you. This will lead you to Rancho Sauardo.
   
   1. Turn around and go into the foyer.
      
   2. Head straight through the next set of double doors.
      
   3. Head through the door on the right.
      
   4. Enter the hallway and take the first door on the left.
      
   5. Head into the kitchen a little bit, you should see a table with two bench/booth seats on the left. Hop on the table and look up to use the portal.
      
4. Head to the roof of the Blacksmith. This will lead you to Downtown Arachnitopia
   
   1. Head out through the door on the left.
      
   2. Head into town and walk until you get to the blacksmith. It's the last building on the right.
      
   3. If you have a jetpack, use it to fly up to the top and use the portal.
      
   4. If you do not have a jetpack then go to the right side of the building attached to the blacksmith.
      
   5. Go up the stairs and enter the building through the door on the left.
      
   6. Head straight through the doors on the other side of the room and go to the right. Walk to the far end of the balcony and jump/crouch your way onto the blacksmith roof and use the portal. I find was a little finnicky for me to do this so I recommend the jetpack.
      
5. Find the tallest skyscraper, which has a portal on it, go behind it and destroy a different skyscraper, the portal is inside. That will lead you to Spider Hell
   
   1. Immediately turn right then follow the roads until you get to the bridge.
      
   2. Cross the bridge then continue around the right of this island until you get to another bridge.
      
   3. Cross the bridge and head straight through the four-way intersection at the end of that bridge. Follow the road as it leads to the left.
      
   4. Take the second right and once you get to the three-way intersection turn left and destroy the building. It should be glowing green. The portal is inside.
      
6. Pick up the MFG so you can squish and stop, until it is done.