This is how you can take videos with your DLSR (or any digital camera), do some editing with a non-linear editor (iMovie, Apple FCPX, Adobe Premier. Movie Maker and a host of others)
After shooting a video and doing some post process editing you can export the finished product to your computer - making sure to keeping the length to 1 minute or less. From there you will need to migrate the video to your smartphone. Depending on your phone, there are several different ways. I have an iPhone so I will either use Dropbox or send it via Apple Messages. Once you have the video on your smartphone its just a matter of posting to Instagram like you normally would do.