If I am a person that has done very little exercise lifting a heavy weight with my left arm, how strong do you think my left arm will be? It is likely to be weak, as you can imagine.
The same applies with the ‘no’ muscles. If you have used them very little, they will be weak. They will be awkward. They feel feel unfamiliar and uncomfortable. So know from the get-go that you get better at saying ‘no’ the more you practice it.
So start small (with the less important ones) and build up to the big ones. Every repetition counts. Practice saying ‘no’ in front of the mirror. Remember a time when you said a very firm ‘No’, and how your body language and tone were. Then replicate that. Or better yet, watch a session of Cesar Millan, The Dog Whisperer as he shows people how to be more ‘assertive’. It is all about body language and energy. Say ‘no’ from there.