Easy, a droplet is nothing more than an automated action.
Step 1- you have to create the action. In Photoshop create a new action with all the steps you want to achieve (fit image to 1200x1200 > 150dpi (or whatever you like) > add watermark > export as PDF, ect) and save the action.
Step 2 is to create the droplet. On a mac, go to: File > Automate > Create Droplet. In the first menu, "Choose" a name for the droplet and save in a favorite spot. I put mine in a folder called: Production > Droplets. This folder always stays in the same place, so if you change it, then you have to tell PM where it is again.
Step 3, "Play," you select the action you created in Step 1. Depending on how you run color management, you can check the boxes for suppressing open and profile warnings.
Step 4, "Destination" folder for the resulting droplet that you are creating. It is a good idea to create some destination folders and leave them in one spot on your drive so they will be easy to find. In my case, I have in my production folder, folders for: tifs, psd, jpg, web, etc. Click OK and your done.
Final step is to tell PM where your droplet folder is. In PM open the Preferences > Launching > Photoshop Droplets Location... and you are done.
Now when you have your contact sheet open in PM and you have selected whatever images, just go to the menu Image > Send Photos to Droplet > your droplet... and PS will run the action (droplet) and put your resulting files in your predefined "Destination" folder.
Dale