Just start talking. Learn to auto-speak.
And record yourself on video. Listen. Look for your strengths and weaknesses.
To help you: https://www.youtube.com/channel/UCixkVo3hOENGFlkCemE76Tw?view_as=subscriber
I don't know if you mean verbal or written expression, but the advice in any case is this: write and re-read. Re-read what you have written and analyze - can you put it more shortly without losing your thought? If possible - cut mercilessly. There is no need to drift away and stories from the creation of the world.
If the same thing can be expressed in different words, think about which word suits best. "Beautiful" is a neutral word. It's about anything. Charming, attractive, chic, pretty - that's about different things. Which synonym best reflects your idea? Choose the most appropriate words.
Reread what has been written after a while, with a fresh eye - do you understand your idea? Is it expressed exactly? Do you have an introduction, thesis, facts proving the thesis and conclusion? Have you convinced yourself? If yes - done, you are amazing. If not, go back to the beginning of the instructions and continue.
Being able to express thoughts on paper accurately and in a structured way, you will be much better at expressing them in life.
On mistakes. This is generally the most effective way of teaching now, which is proved by the victory of the Alpha Go system over Lee Sedol. This system literally learns from mistakes.
Here you can recall that Fyodor Ivanovich Tyutchev once described the task of adequately expressing thoughts as completely unsolvable
How can the heart express itself?
How can another understand you?
Will he understand how you live?
A spoken thought is a lie.
By blowing up, you will disturb the keys , -
Eat them - and be silent.
Apparently, Tyutchev's interlocutors did not always understand correctly.
Fifteen years ago I translated from English a book on how to seduce women. This is also an extremely topical question. So, the main postulate of the author was this - you need to regularly make attempts and not worry about defeats. Failure should not be perceived as a final loss, but only as an inevitable price to be paid for the experience gained. It seems quite obvious that if you only dream about yourself, but never tell the girls how beautiful they are, then you will definitely not learn anything. So it is with the expression of thoughts. It is necessary to minimize the experience of failure, if any.
Firstly, you will have to refuse to give marks to yourself - "good", "bad". Second, express. With words. Experience and conclusions from experience can only arise in experience, not in "learning" (in advance).
You cannot learn to swim from books or advice from swimming experts. You can swim and express your thoughts only by doing this - to swim, to express. And learn from your own, real experience.
Perhaps the best way to learn to express thoughts well is reading (preferably thoughtful). It expands the vocabulary, and this is the most necessary for expressing thoughts. Logic will also help a lot (in order to connect one with the other). Finally, think. I have situations when a stream of incoherent speech breaks out, but after a pause and thinking over all the information for five to ten seconds, I express myself clearly and in a structured manner