Start with an environment, where you stay more or like it more and need to change.
Think about the colors you would like this environment to have, according to your style, etc...
do a cleaning
Discard everything that is no longer needed, and that way you will be more aware of how much space you have.
Make a list of the things you will need to compose the decor, such as pillows, curtains, rugs, etc...
make a list
Measure the environment in question, to find out how much space it has, and what needs it will meet in that environment.
Try to maintain balance in the composition, do not put too much or too little, focus on the essential and fundamental
Create layers of textures, color shades, this process is super fun. So have fun!