1. First, sign up for a free account on RapidAPI.com
2. Search for the SpellCheckPro API in the marketplace and select it
3. Select the desired pricing plan and subscribe to it
4. In the API documentation, check out the endpoint and the required parameters. Please make sure X-RapidAPI-Key is one of the header parameters.
5. In order to test the API, you can use the test button in the documentation.
6. To check the spelling of a word/sentence, you need to pass the word/sentence in the parameter ‘text’ and language code in the parameter ‘lang_code’ in the API call
7. The API will return a JSON object containing the misspelled words in the text.
8. You can use these misspelled words to correct spelling errors in your application.

Please let me know if you need more information.