1. Always start with function
List the key functions of the room first - who will use it, how will it be used, how much time will you spend in the room. For example, if you're redesigning a living room where watching TV is the main function, make sure you give top priority to the placement of the sofa so that you get the best view of the TV - ensure the sofa you select is comfortable rather than just good looking. If you're designing a bedroom, worry less about the style of the occasional chair and focus on understanding how to maximise your storage space. Maybe the spot where you currently have your wardrobes isn't necessarily the best option.Make a list of what each room's main function is and crack that first - the rest then has to file in behind. Of course you will make compromises but make sure that you weight them in order of functional importance not aesthetic importance.
2. Sketch it.
Do a rough sketch of your room. It doesn't have to be architectural but make sure you include plug points, radiators, window sill heights and any other fixed items on the drawings. If there are lose bits of furniture that you are 100% certain you'll be keeping in the room, draw them in too. Measure everything in cm rather than inches - its easier when selecting items online or in shops. if you're feeling up to it, try using a free drawing program like Sketchup to get more accuracy - https://www.sketchup.com/
3. Make a Spreadsheet
It's a quick and simple way to keep track of what you need and what to budget. Make a list of all the items you think you will need; include smaller items like paint, cushions, lamps, vases etc. and then assign budgets to the items individually.
You can later move budgets around but it's good to start with a budget (even if you adjust it as you go along.
4. Sofa, dining chair, mattress
Allow a good % of your budget on these 3 items. They directly affect your enjoyment of your space and have an impact on your physical wellbeing. Better to spend more on a good quality sofa that will give you good support and last for many years rather than a good looking sofa that isn't 100% comfortable.
Don't buy your sofa on the first sitting. When you've decided on a model, sit on it for at least 20 minutes in the showroom - tell the salesperson what you are doing and even take a book with you if it helps kill the time!
Use wallpaper to add depth and contract to a room. Don't just put one piece on a back wall and call it a "feature wall". Apply wall paper in an L shape so that it goes along at least 2 walls and then compliment it with a paint colour that is present in the paper. You don't have to go for a heavy pattern or a loud colour - if you like softer colours, go for a paper that has a very light patter or even no pattern - wallpaper adds depth to a room in a way that paint just can't achieve.
6. Don't hang your TV at painting height.
Your TV is not a painting. The best viewing angle dictates that your TV should be hung no more that 1.5 meters from the floor. Also avoid hanging TVs above fireplaces or above anything that distracts attention.
Don't hang artwork over your TV either.
7. Trust your gut
Take advise. Read what designers have to say online. Listen to friends who's homes and style you like.
In the end though, obvious as it is, go with what you like and trust your gut.