Pick a combination of clothes that you would wear for a long walk on a "typical" day at home, assuming a sunny day with the temperature starting at 8C and going up to 25C. That will probably include:
- light base layer with short or no sleeves,
- light layer with long sleeves to protect from sun and give a small amount of insulation,
- light fleece or jacket in case it stays chilly.
That is 3 light layers, for typical walking when it is not raining. As the day warms up, you take off layers.Then consider what you need for rain - if your layer #3 is waterproof, you don't need a different one.
You should also have something warm and cosy (be sure to keep it dry) to wear in the evenings. In case of "emergency" (really unexpected cold weather) you could wear it in the day.
You mention a "flannel shirt." I'm not sure what you mean by this, but don't bring a cotton shirt unless you intend it to be to keep you cool on very hot days. Otherwise cotton is heavy, slow to dry, and not good insulation.