The goal of a motivation letter is to present yourself to the host NGO.
I like to keep it short (no longer than one page). People don’t have the time to read novels when selecting a volunteer.
You should include the introduction and closing statement, while the motivational part I tend to divide in these three simple smaller bits.
1. Say something about the NGO and/or project that interests you. Explain why and connect it to your experience.
2. Say something about yourself, what motivates you, why you decided to do the EVS in first place.
3. Say how you would bring added value to their project, how you would fit in their proposed activities.
I wouldn’t make it more complicated than this. This is what people want to find in the motivation.
Check this guide as well, I found it pretty useful.
And good luck!