How to design blog? It has to be learned by doing it practically or watching any related YouTube video for designing blog.
Login to your blog account
Go to Design / Setting option
Now, you will get the option of various blog templates at the bottom or when you click on 'Template' option at the left side or above 'Settings' option
Select blog template per your choice (some template will not be able to change the theme or design or other required setting per our choice)
Now, click on 'Post' or 'New Post' or 'Create post' (click on Pen icon) option to write something as topic, enter Post Title per your choice
Enter summary at big blank box
Use the various option above big box respectively
You can use bold tag, Italics letter, underline, different font, different font colors, different highlighting colors, insert image, insert link, insert video link, left or center or right alignment of the text etc.
Once everything is done, click on 'Publish' or 'Save' option to publish it and SAVE its URL for future usage.