A few points that might help you out.
1. On your user profile, yes, you must turn off the checkbox that reads “Disable the visual editor when writing”. Having that turned on will disable the block editor.
2. If you’re using the Classic Editor plugin, then on the Settings->Writing page, there is an option labeled “Allow users to switch editors”. Set that to “Yes”. When you do this, then on the Posts listing screen, each post will have “Edit (Block Editor) | Edit (Classic Editor)” options on it, along with telling you which editor the posts are currently using. You can use either editor for any post, and nothing will change about your post from doing so until you click the “Update” button in that post.
3. Make sure you have Revisions enabled. Or rather, make sure you don’t have a plugin or setting to disable them. Revisions work great and will let you revert any changes you make in case an editor screws up your posts.
4. When you are editing a post in the Classic editor, then there will be a Visual or Text option in the upper right corner. Switch it into Text, and it will remember that you did that, and posts should go straight into the text editor when you use the classic editor.
5. When you’re using the block editor, there is an icon in the top right of the screen that looks like three dots vertically. That’s the big menu. In there, you will likely want to turn off the Fullscreen mode, and you can also use this menu to switch to the “Code Editor”, which basically shows you the whole post with the block markers and all in a simple text box form.
Just try it out for a while on a test post if you like. You’ll get used to it, and pretty soon you’ll be wondering why you didn’t switch to blocks sooner.
