Hi there :)
I'm not sure if you can post such requests here.. but I'll jump in and offer my two cents :)
Apologies to Mods, if this isn't allowed (as I suspect)
When I was at school and had to write such essays, I always used to find it useful to make sure I include all the five senses in my descriptions, where possible. This helps make sure that you don't miss out on any aspect of the thing you are describing.
I think it's a good technique, especially under exam conditions, because it gives you a framework with specific questions to ask yourself -
1. what does it (the thing, person or place you're describing) look like? (colour, shape, other attributes - shiny/dull, )
2. what does it smell like? (it's not always easy to find words for this category, so a good idea would be to think of a smell associated with another thing, eg, chocolate-smelling)
3. what does it feel like? (hot, cold, hard, soft, etc)
4. what does it taste like?
5. what does it sound like?
Of course, you won't always have answers to all 5 questions for every single thing that you are describing. For example, if you're describing a dog, you can't say what it tastes like. And there's no particular smell either, Saying "the dog smelled like a dog" isn't particularly clever. lol.
But the idea behind this technique is to just think about the thing you are describing holistically.
Also, in descriptions, never forget to include what you feel, ie what a certain thing, person or place makes you feel, where applicable. So it can be a "gloomy day," a "happy song," or a "terrifying laugh."
Imagery is a very good idea, especially where you cannot find adjectives to capture exactly what you are trying to say. Imagery, used well, can be very powerful and succint at the same time. I can think of three things you can do:
1. simile - saying a thing is "like" another thing, or comparing it in terms of another thing.
eg. "He ran like a cheetah"
2. metaphor - similar to a simile, but you don't use words like "like" or "as".
eg. "Life is a race... if you don't run fast enough, you'll be a broken egg" (3 Idiots) :)
3. personnification - describing an inanimate object as though it were a person,
eg. "The tree smiled at me wisely," or "The bed groaned under the weight of the books."
As far as feelings in particular are concerned, I can't think of anything but trying to imagine what emotions certain situations would invoke in you. You'd need to put yourself in the world of the story and imagine what it feels like.
Imagery is a useful tool, because then you can say things like "I felt like my heart was being hacked to pieces." You can be imaginative and come up with beautiful pieces of writing.
Another good idea would be to think what a certain emotion does to you physically. For example, think what happens to you physically when you're very happy. Your face lights up, your eyes leak with happiness, etc. When you're nervous, your palms are sweaty, your heart pounds, you shiver, your knees shake, etc.
Writing in general - other tips
If you practise writing in your spare time, trust me, that can do wonders! Writing creative stuff under exam conditions isn't easy. But if you have practice, words just flow out without you having to think too hard.
Also, try to read a lot and widely. It really opens your mind to new ideas and ways of thinking. When you read, don't just focus on the story. Notice what the author does. If a particular sentence strikes you as beautiful, ask yourself why. What is it that the author does that makes the writing so powerful.
I used to have a vocabulary notebook for all the new words I learnt. Having a rich vocabulary definitely helps!
All the best!! :D Be confident, believe in your abilities and you'll be surprised!
Ps. It's "writing" not "writting" :)