From the Images page, on the Custom Images tab, open the More menu of the custom image you want to create a Droplet from. Then, click Start a droplet. This takes you to the Droplet create screen with your custom image pre-selected.
Regardless of which method you choose, finish selecting your options on the Droplet create page, and then click Create Droplet. You can only create Droplets in the same region as your custom image, but you can add custom images to other regions.
After your Droplet is created, you can connect to it with SSH.
If you get a prompt to enter a root password and you don't have one, check the version of cloud-init on your image and make sure it's at least 0.7.7. You can also try adding your SSH key to the image before you upload it.