the easiest possible way
There are several alternative processes techniques that we always talk about that have one step in common: creating a digital negative. Once only possible on film, now, thanks to digital photography and photo-editing softwares, it is possible to make negatives out of any image, whether you shot it on your phone, digital camera or downloaded it off the internet. Below, some tips about how to get it done (you can do it on Photoshop or GIMP, GIMP is completely free and the process it's pretty much the same)
What you need:
a digital image
a printer (whether it is laser or inkjet will depend on the kind of acetate you get)
In case you have Photoshop installed in your device of choice, the steps are the following:
First, open the picture you desire to print. Then, go to Image settings and select Adjustments and then the B/N tool
After making the image black and white, it is possible to modify the brightness and contrast as well. For this, go back to Image settings, Adjustments, and select Brightness/Contrast. For cyanotypes or photograms on fabric it is especially important to have a higher contrast to achieve a better result.
While in the Brightness/Contrast window, to achieve an even more dramatic effect select the Legacy checkbox and bring Contrast up and play around with the Brightness.
The end result is how your print will look like. However, in order to print it, you will need a negative. For this, return to Image settings, Adjustments, Invert.
In Photoshop, you can also choose to edit the size of your image to make it fit the area you will print it on.
If you do not have Photoshop installed on your computer, you can download GIMP. There are also free photo editing apps inside the Apple Store and Google Play that can do the trick. For example, Snapseed is a free app that has different editing tools that will allow you to do all the editing in your phone before printing.
Once the editing is done, you are ready to print the negative. If you are printing at home, make sure your type of printer (inkjet or laser) is compatible with the type of transparency sheet you got, otherwise the ink won’t stay. Follow your printer instructions, and print away.
After printing be careful handling the transparency since the ink can be scratched. for this reason it is wise to save it between pieces of paper to protect it. The cool thing about printing your own negatives, is that you can play with the image sizes, shapes, print your own text, even make collages.