I had a room which was spare so I decided to make use of it and made an escape room for my friends to play.
A concept or a theme is a good place to start. Pick theme and go (eg. crime solving, horror, medieval, prison, etc.). Mine was defuse the bomb in a time limit by entering a password. If you have limited resources have a look around the house and see what you use. There are plenty of items that you have never used in years that could be used (board game pieces, locks, costumes, gadgets, etc.). Once you have assembled a bunch of different items start brainstorming. Write down any and all ideas.
Then start on a flowchart. It is the easiest way to map out your escape room and see how everything fits. Take those ideas you wrote down earlier and see how they can fit together.
Here is the flow chart of my escape room. I will explain it below.
You don't need to understand the contents of the flowchart. Focus on the structure. The main thing to note in the flowchart is it convergences on the 'Input password to defuse bomb', a single overarching goal. That is what you want for your escape room. It will need a clear end goal. It could be something like find a key or a code to unlock the door to escape room (don’t restrict yourself to just those options).
The other thing to note is the number of branching paths representing different puzzles. This gives the players a variety of things to do. If they happen to get stuck on one thing they can focus on something else and go back to it later. My advice is to give the player some freedom to choose and make them build towards some end goal. Funnel them towards an end goal. You will see that in every escape room you will come across.
Map out the puzzles and how each part relates to each other.
Give the player multiple things to do so if they get stuck they can do something else and come back to it.
Clear end goal players are working towards.