When you modify an experience in Curator, you will be provided a list of pre-made templates that you can modify. 

In your 'Customize Experience' tab you will be prompted to choose a new number of frames (1-4). This will reflect how many photos will applied to the template. If you choose 3 frames, you will be given all of the premade options that contain 3 photo holders, as shown below:

Once you have selected the specific photo layout you want to use you can either leave it as it is, or you can modify it. In this example we have chosen a classic 3-photo strip. In order to modify it, click 'Download PNG'. You may also choose 'Download PSD" if you are proficient with working with layers in Photo Shop. 

The PNG or PSD file will download. You can then open this file in the program of your choice in order to add verbiage, logos etc. 

Once you have modified the file, save it as a PNG (regardless of which option you initially chose) and then upload it to the curator site using the 'Upload Custom Design' button in the screenshot above. 

Your template will now reflect the changes you have made.