You just have to log in with your username and password, and then select the picture you want to upload. You can't do this with any of your pictures, it has to be square and have a minimum resolution of 640x640 pixels, matching Instagram's usual parameters.
From the interface in Gramblr you can see a recommendation for a website that makes your images square, even though you can do this with any photo editing application. Once the picture is ready and in JPEG format, you have to press on the Upload button.
After this step, you just have to insert a short description that appears beneath the image. Once uploaded, the options are the same as in the mobile version: you can share the image on Facebook and Twitter.
You can't use any effects, so you have to accept a result free of filters. Still, it's a good option to upload pictures from your computer, since Instagram's web version doesn't allow you to.