Step 1 - Tools
As with many graphic design requirements, I use PowerPoint to collect images and I want to use and manipulate them into a collage of text, shapes and pictures. You may also want to check out canva, at least for inspiration or to throw something together quickly, but in my opinion it is not ready as a tool for complete creation yet. My recommendation for Pin-able graphics is to start with a blank presentation and use the Design ribbon to set the Slide Orientation to Portrait (longer vertically).
Step 2 - Sizing
Limit yourself to about a third of the page. If you don't have Guides and Gridlines activated, right click on the background to do so. Set the vertical guideline to 1.25 to the left of its default center position (drag it to he left until it says about 1.25). This little box is your working area.
Step 3 - Search
I use Google images search to find the images I need, or at least inspiration. If you plan to overlap a few simple images, you may want to use the Search Tools
to limit your search by Color to Transparent. What this does is essentially filter on images that Google thinks has a transparent background - its not always perfect but it will get you better results in general.
Step 4 - Crop, Remove Background, Rotate, Manipulate
Once I find an image, I copy it and paste it into my PowerPoint slide. Then I may want to crop the image or I may need to do some processing to remove the background. If there is a background that you want to remove, the most recent versions of PowerPoint have a powerful Remove background function under the Picture Tools ribbon (note, you must have the image selected for this ribbon to be visible). Older versions of PowerPoint have a Set Transparency Color function available, in which you just select the background color and it makes it all transparent. I'm also a fan of rotating graphics slightly to give them a more exciting look. Sometimes keeping them straight portrays a more sleak, streamlined look, which may also be what you're trying to accomplish, so play around with rotating and decide which will work best for you.
Step 5 - Keep Building
I repeat this until I have the images I want to work with. Then I add text to go along with the graphics. Consider putting the text at a slight angle with the rotation tool, especially if you left your graphics straight. Or add shadowing or outlines to the text to make it stand out from the graphics.
To make really long pins in Pinterest, build down that section we created to the end of the slide. I caution not to make the graphics much longer than this, as really long pins tend to be annoying to users and will graphically be shortened
so you lose part of the graphic if it gets too long. Keeping it about the length of the slide ensure that most of your graphic will be seen, and it will take up a nice amount of the screen when viewed on a board or on the home screen of Pinterst users.