Measure the room dimensions as exact as possible, including where the water comes in, waste goes out, windows, doors, pipes, boiler, currently electrics/gas supplies etc and plug it all into an app.
Units and appliances tend to come in standard sizes - 600mm, 500mm, 400mm etc so you can use what you currently have as a rough starting point even if you do shuffle about a bit and play about with what looks best in the app.
Be aware of limitations such as not having oven right next to fridge or door/wall or in front of window, extractor fan will need outside vent, waste pipes can be difficult to move etc.
Then take to a couple of places for a quote and ask for their advice etc.
Once you learn more about it you will be able to judge if you think you can do it yourself. Remember things like walls that are not perfectly plum, straight, flat (most walls!) can make fitting much harder and usually need experience to get a good finish.
Remember to leave space somewhere for a bin!