To make a post with Page Builder please follow given below steps:
Step-1 : Go to Posts -> Add New.
Step-2 : Give post title.
Step-3 : Click on Swift Page Builder. After clicking it you can see the code area where you can put your page design.
Step-4 : Click on Choose Element and select an control from that options which you want to add in your post.
Step-5 : You can see the selected control on Given code area, just make design into it as you want and Publish the page.