You can also do stuff like use cheap folding chairs and tables until you decide on real ones, mark out the dimensions of furniture on the floor using masking tape, drape fabric samples on things to help you visualize the final furniture etc.

Some other design principles to keep in mind is:

• visual weight & balance - by default a visually balanced room will feel more comfortable, and this is determined by a somewhat nebulous feeling of visual weight. i.e. if I put a large black tv in the south west corner of the room, I might want to try and balance that out with a large piece of art, or a bookshelf or something in the north east corner.
• contrast - part of weight is not just absolute colour, but also contrast. A heavily contrasting design element (a black tv on a white wall) feels much heavier then a black tv on a dark green wall. Contrast tends to draw your attention so it can also be used to pull your gaze and draw attention to things. A painting with a large blank wall around it stands out and draws your gaze more than one on a wall filled with a bunch of other elements.
• proximity / hierarchy / alignment - proximity, hierarchy, and alignment all create feelings of things being connected together, and can be used to imply relationships. If the top drawer has your most used cutlery & utensils, the drawer below it will often have the second most used ones. This is practical from a "grabbing closer stuff more often" standpoint, but that layout's proximity and hierarchy also make it self explanatory and intuitive.
• cognitive overload - people get overwhelmed with too many options, when organizing it's helpful to compartmentalize things so that people don't just have to choose from all options. A grid wall with every option always visually available is overwhelming. Having sections with doors and labels mean you just have to know which section you need, and can then select from a smaller number of options after you open just that door.

And most importantly to interior design is lighting. I cannot stress enough how important good lighting is for making a room look good. A well lit room can make any room look kind of decent, and bad lighting can make any room feel uncomfortable.

People, at a base level, don't like not being able to see areas of a space, like areas that are in shadow. It is a deeply engrained survival instinct because that's where predators hide. So if you light a room from a single point source like a single ceiling fixture, you will have a bright spot in the center, dark spots at the edges, and shadows cast anywhere between objects and the wall. This is the worst way to light a room.

What you want is lots of sources of diffuse light. Lots of shaded lamps from lots of angles will create a much more comfortable space. It's also why people like those ceilings covered in a grid of pot lights. That grid of tons of pot lights creates a ton of different light sources. It's not perfect on its own though, it still creates downwards shadows, and will still benefit from diffuse side lighting like lamps. And I keep stressing shades and diffusers because glare is just as bad as shadows. If a point source is too bright, your pupils have to contract to see things near it, and this suddenly makes everything else in the room too dark. You want soft, even, indirect lighting.

When it comes to the importance of lighting, remember that everything you see in a room is light. You don't see the objects themselves, you see the light that has bounced off them. Lighting effects everything.