To upload your posts in bulk via a spreadsheet, open the composer.
In the navigation bar a the top of the page, you will see a "BULK UPLOAD" link:
Click that link to open the Bulk Upload panel.
Then click "DOWNLOAD SAMPLE EXCEL FILE" to get a pre-formatted Excel spreadsheet with examples of all post types and instructions in the sheet.
Follow the directions in the sheet to add your posts.
You can add any post type, but if you are uploading media posts (images, videos or gifs), you need to have a public url for each media file — and the url must end in the supported media file: .jpg, .png, .mp4, etc.
Once you have your sheet all filled in and ready to upload, drag and drop the spreadsheet file into the drop area.
Or click "CHOOSE FILE" and upload it that way.
You're almost ready to go.
Re-check "Posting to" at the top of the page to make sure your posts will be uploaded to the profiles you intend.
Then make sure all your required Network Settings are set (Pinterest boards, etc.).
The color on the left of the Network Settings box should be yellow or green — and NOT red.
If everything looks good, click the green "BULK UPLOAD POSTS" button!
You will see popup notifications for the progress of your upload.
When it's complete, head to Publishing > Planned and check your planned posts.