Had the same problem.
Ease yourself into hand writing, jumping right into it can be daunting.
When you want to type a practice essay, turn off spelling and grammar checking as this seems to be the main issue you stated. This will prevent you from getting that automatic assistance and will encourage you to develop your language and editing skills as you write. You can then turn it on again, and I believe it will highlight as normal what may need correction.
Once you get used to that, assuming you type way faster than you write, reduce the amount of time you have to type an essay. For example, instead of an hour to write, make it 55 or even 50 minutes. This gives you some buffer time for when you transition into hand writing. This technique can be kinda hard to apply, and a lot of trial and error is required; if it doesn't seem like it will work, ignore this!
Not sure if you meant 'using synonyms' as in using the inbuilt synonym function in Word, if you did, avoid using it at all. If you have to, go back after you've written the paper to see what other words you could have used, and try to incorporate it into your vocabulary, but do not try to force it, if you get what I mean (too many people I knew would try to use all these fancy synonyms at the expense of coherent writing).
If you could give more specifics about differences between your typed and handwritten essays, I'm sure plenty of others along with myself would be happy to further recommend some ways to help you! You can definitely improve your handwritten essays before the exam comes around!