How to teach myself to improve my English writing skills
Use Merriam Webster online Dictionary and Thesaurus, which has an audio providing the pronunciation also.
Listening well and reading intently are the first steps in good oral and written communication.
Build your knowledge of grammar (Wren & Martin), syntax, vocabulary and so on.
Before you use a particular word, say beautiful, check the online thesaurus for synonyms and use the most appropriate word.
Vocabulary is built brick by brick. So learning one new word leads to learning many others. Check the exact meaning of every new word from the dictionary, instead of trying to guess the meaning from its context.
Read a lot of good authors. Listen to a lot of good speakers. Yet, in either case, develop your own unique style.
Write short speeches, memorize them, internalise them and deliver them before the mirror or a better speaker than you. Practice makes perfect. Join a local Toastmasters Club or other public speaking club.
‘Standing Ovation -Fifteen speeches to steer the budding orator' by Sterling Publishers on Amazon will be a great help.