How to avoid major spelling mistakes

There isn't any quick solution to this - no magic wand. But here are three suggestions.

1. Check words in the essay title especially carefully. Other words to check are technical words, and words which are the names of people or places. Make absolutely sure that these are right. Then...

2. Add these words to the Spellchecker on your PC. If you can't do this, or you're not using your own PC, then...

3. Learn the word. Like this.
Look at it until you know it. Say it slowly to yourself.
Without looking at it, write it out (I mean write it by hand).
Check that you got it right.
Then write it out several times. This is so you practise getting it right.
Write it in different ways - write it big, then small, write it in different colours, in capitals as well as small letters.
Finally, check that you've got it right.

OK, this all sounds a bit childish, and you might want to wait until you're on your own before you do it. But it works. At the end of the process, you will know the word with your ears, your eyes and with your hand.