First, find a vector program that you can measure out the size you want your postcards to be. I roughly made mine 6" x 4 1/4". Make sure you have rulers on and set them according to the document. This is what your postcard may look like when you are finished.
Print your postcard out on card stock. This will be your postcard frame which I will talk about later. Now, select all and copy it. After you have done that, open a new document and paste your postcard on it as many times at it will fit on the page. As for the page offset, you won't have to worry about it. I measured the postcard size after printing it out and it's still the same. The black lines that you use are only a cutting guide anyway.
Second, you need to make a postcard frame. With this, you can center the pictures you want to cut out. Take the printout from earlier and using an xacto knife cut out the postcard making a frame like below. Use this to frame the picture you want to glue to your postcards, then using a pen draw a border around the picture with your frame to cut later.
And for a little extra flair, you can make an envelope for your postcards! To do this, make a new document and past your postcard template into it. Then, make shapes that overlap like an envelope over the postcard template. This is how you'll see what will fit and how you can glue it. (Note: Copy and paste the shapes that are similar and rotate them so that you have perfect shapes for each side). After you are done doing that, move the shapes to the outside of the postcard template like below. Make sure that you put enough space between your postcard template and the outer pieces so there will be enough room for the postcards to fit inside. Otherwise they will be too snug. You can then print it out on card stock or paper and fold along the lines!
Thank you for reading this post and I hope you enjoy making postcards!