The screen process I will show you uses EZ Screen, a product I love and have had success with using. I recommend getting their starter kit for all the items you will need during the process. Paul had my back, I've got yours too; use coupon code PAUL15 to save 15% on your order.
First, adjust and prepare your images using Photoshop. Essentially you need to simplify your image to a black and white subject, eliminating shadows and color variation for your screen to be successful. If you are using text or a type of line drawing you should be set, aside from resizing if you want. If you are using a photo first adjust your image to gray scale, then use the photocopy filter. This will simplify your image into a line drawing. From here play with brightness and contrast to bring out or cover up parts of the image. Don't be afraid to use the erase tool to get rid of unwanted details, it can help clean up the image nicely.Crop as you wish, and create a document with your images. Be sure to leave at least 1/2" of space around each image (there should be 1" between two, 1/2" for each). Print off a page and you're ready to move on.
The next step is to make a transparency copy of your image. Some home printers can do this, check into yours before attempting to print off at home. Also, from what I've seen it is incredibly expensive to buy a box of these pages, though maybe I'm searching in the wrong spots. I generally take my images to the local UPS store or Kinko's, and they tend to run about a dollar a page. I strongly suggest making two copies of your image on transparency pages, as much of the success will rely on the opacity of the ink.
You're ready to make your screen so get out your EZ Screen kit and transparency images. Line the transparency pages up together, and make sure there is no light visible through the ink. To secure my pages together I use a small bit of tape on two sides of the page away from images so it doesn't disturb them. Make sure the room you are working in is dim before pulling out your screen, as the EZ Screen is light sensitive. Slowly peel away the protective plastic layer from the EZ Screen and put the screen on your black board, shiny blue side face up. Now place your transparency pages on top of that, so that your images appear reversed when looking down at your project. When you go to screen your image, the blue side will be face down, making your images appear correctly. Place the plexiglass on top and clip the project together, making sure you aren't covering the images with the clips. I like to cover the project with a towel at this point to help block light while I transport it.
To create your image in the screen you may use a lamp or natural light. I've always worked with the sun on bright days and it's been great even though sometimes the New England rain can hide the sun for days. I like to find a spot in full sun, where I'm sure the shadows won't adjust my way. Remove the towel to expose the project to light and start a timer. At the Wandless workshop he gave a time frame of five minutes in bright sun. The time given in the instructions that comes with EZ Screen is different and I've never tried it, I've only used 5 minutes in sun. When the time is up recover your piece with the towel and head to your sink.
Again, make sure your working room is dim. Take apart the project and place the screen in a sink or bucket of cool water for at least 15 minutes, though longer will not cause harm. At this point, I usually cover up the sink with boards or towels to block the light, so I can flip the lights back on and work on something else in the studio while waiting.
Now come the final steps. Place the exposed screen on the plastic mesh screen supplied in the kit. Rinse the page under running water. The material on the screen that was under the ink of transparency pages remains soft, washing away creating your image to screen through. The rest of the material hardened after being exposed to UV light and the soaking time. If there are portions that won't wash out you can carefully rub them using a sponge or soft bristled brush. If your image will not wash out, it may be overexposed. If everything washed out, it’s underexposed. You will need to start with a new screen if either is the case. Once clean, put your screen back in the sun for another 20 minutes, to further harden the material for better durability.
The final step is to cut out your screens, leaving that minimum 1/2" border around each image. This is the space you will hold the screen to your work, so adjust to fit what's comfortable for you. Don't store your screens together in a stack, they can stick together. Instead store them between pages of paper.
I've used the screens successfully with slips and underglazes, on wet to bone dry clay, but I find the best results come from using underglazes on leather hard clay. When I use the screens I like to dip them in water and pat them dry with a towel. Wetting them softens them, so they are better able to hug the curves of your pot, and creates a better seal in my opinion, but only if you have patted them dry. Generally you don't want your underglaze or slip to be too watery or too thick, each causing issues. As many things in pottery go, this isn't a rule. I've had very watery underglazes give fantastic results. I also have a particular jar of underglaze that runs terribly, no matter how I adjust the consistency. So as always, experiment!
I really like this technique and have used it for various projects over the years, but I have used it much more recently, screening chrysanthemum drawings on trays and other items. The blue on the white reminds me of Chinese cobalt work.