To add and edit new blog posts go to Entries > Blog. Once there you can edit an existing entry by clicking it's title or creating a new entry by clicking the New Entry button at the top right.
There are two different types of blog posts Blog and Legacy Blog. Legacy blog is for older blog posts that were migrated using the original field options for the old blog. Blog is for new blog posts and includes the new page builder option.
If you want to, you can edit old blog posts and switch the type from Legacy Blog to Blog. This will however involve re-entering the content into the new page builder fields. You can change entry types by selecting the entry type on the right hand side as indicated in the screenshot below.
Changing entry types will show different field options.
When you create a new entry with the type of Blog. You will have a series of fields to fill out. At the bottom of the page, there is a Blog Content builder with a number of options. Each of these options is for a pre-configured content type.
Clicking each of the buttons will reveal the fields needed for each of those content types. For example in a test entry I created a Gallery and have added content to the three fields.
Here you can see the Gallery Title, Gallery Description, and Gallery Images fields. You can easily add more images by clicking the Add an asset button or remove an image by clicking the X next to each image.
You can re-order the images by dragging and dropping them. Additionally for the Gallery it is important to set the image width for each image by double clicking the image and choosing from the three options available. Here you can also edit the Image Title which is used as the caption of the pop-up on the entry page.
In the image above you can see the image size options. You can also see for this image that the Title of image is by default the same as the file name. You'd likely want to have a more descriptive caption. Change the text and save.
You'll also want to set the focal point for images to ensure that the most important part is visible for the auto generated thumbnail.
In the image above you can see that each content block has a gear icon which when clicked will reveal additional options allowing you to collapse, disable, delete or add other blocks above the current block. Collapsing a block will hide the fields.
Disabling a block prevent the content from displaying on the website, but save it in that block so you can bring it back again in the future. This adds a red circle indicating that the block is disabled.
Next to the gear icon is a handle that you can use to drag and drop blocks. This allows you to easily reorder your content around the page as needed.