What to do if you open your front door one day and there is a portal to an alternate dimension instead of what you expected to see, which was probably something along the lines of whatever is usually outside your house
Get something that's not very valuable. Like a large rock. Throw it into the portal. If it goes in, you know that that is actually another universe and not an illusion caused by the Head Bad Guy.
Observe its descent. Does it fall and hit the "ground," or does it just keep falling?
If the rock vanished immediately, it is indeed an illusion caused by the Head Bad Guy. If you know his name, shout it at the top of your lungs and challenge him to a duel. Then all you have to do is win and everything will be back to normal. If you don't know his name, try Bob.
If the rock continued to fall without hitting anything, it is obviously a bad idea to enter as wherever that portal leads, it is very high up and affected by gravity. Proceed to step 3 for cautionary reasons only.
Probably, though, the rock will fall to an invisible ground. Then you need to get something else that isn't valuable, like a stick; make sure that it's long. Hold it into the portal, but don't put your own hand in. If there is an atmosphere in there it probably isn't one you would like very much. Anyway you need to wave it around a little, you know, see if Newton's laws of motion still apply. DO NOT DROP THE STICK. When things seem to be OK, take it back. See if the other dimension has affected it at all. If not, it's time to move on to Step Three. If it has, try to determine whether the stick has been altered beneficially or otherwise, and ask yourself "do I want what happened to this stick to happen to me?" before proceeding.
You need to see if the atmosphere in there is OK for humans. This is the really brilliant part. Get a sealed bag of potato chips. Two if you're hungry. Take the bag to the portal and throw it in. This is crucial. If the atmospheric pressure on the other side is too low, it will pop. If it is too high, it will crumple. In either of these cases it is what is known as a Quasi-Dimensional Wormlike Anomaly (QDWA) and you need to run like there's no tomorrow, 'cause baby, there isn't.
If the bag remains intact, it is safe to enter. Put your arm in first and get a feel for the "air." Then your head. See if you can breathe. Be ready to pull yourself out at a moment's notice. If all is well, tie a rope around your waist, tie the other end to something heavy or bolted down and enter the portal.
Walk. Walk straight forward. Turn around every once in a while to make sure the portal to your universe is still working. If it fluctuates, dart back as fast as you can. Caution is key here. Don't let the rope break.
Eventually, you will run into a person native to the universe, another hole, or some sort of unidentifiable object. This person/hole/object will be benevolent and helpful, according to the Second Law of Alternate Universes. When you do meet them, they will give you further instructions, and my part in your journey is over. Good luck.
However, if you bump into some evil entity, retreat. There's an excellent chance that you will not make it, but hey, worth a try. If they catch you, try to think of something nice and happy while every inch of your body is stitched in the big wall in hell by a blazing needle. Think about how fine your life was until now; and if it wasn't, keep in your mind that is not gonna last longer. Either way, you're gonna go absolutely barking mad, so don't stress out much.
If you don't run into anything in there, you are dreaming, and there is a roughly 97.2% chance that it is Monday morning and you're late. Please wake up.