Open a new Google Slide

Select "Page Setup" under "File"

Click on the box that says, "Widescreen 16:9" and select "Custom:

Enter 6x4 inches and select "Apply"

Now you'll have a blank workspace and you can design, type, insert, or draw whatever you wish. (See the second image)

Once you have a signature that you are happy with, you will need to download the image: File, Download, PNG Image

To use your signature you will need to go into Gmail and open the settings

Select, "See All Settings" and scroll down until you find the "Signature" section.

Select, "Create New" and name your new signature (it can be anything)

After you have named your signature, you will select the image option (see the third image). Then you will choose to upload an image, find your image, and select it.

Once your image has been uploaded, you can click on the image and then you will be given the option of choosing the size of small, medium, or large. You may select any size you want, it will just impact how large your signature will show up on your emails

Finally, you will need to select the Signature defaults that you want (see the fourth image). You just have to select the name of your signature

Be sure to save your changes!! Once you have done all of that, your signature should be at the bottom of each email!