Purchase some paper that is 0.007-0.016 inches thick. If your paper is thinner or thicker than these paper weights the post office won't mail your post card.
Open a blank Microsoft word document. Create a box that is the size of your postcard in Microsoft Word. You can create a square by clicking on "Insert" > "Picture" > "Auto Shapes." Choose a square. Draw a square on your page. The square will most likely be filled. You will want a square that has a black stroke around it with no fill.
To change the fill of your shape double click on your shape. A window will pop up that says "Format Auto Shape." Select one of the buttons below that will say "Colors and Lines". Now you can select "No Fill" for the box. Click "OK".
Double click your box you made again. The same window will pop up that you just used. This time select the button that says "size". Here you will be able to type in the document dimensions. Measurements for your postcard should be no less than 3.5" x 5" and no more than 4.25" X 6". Once you have typed in the dimensions click OK and your box should change to the new size.
Now you can do a number of things to decorate the front of your post card. For this example I am going to write the word "Thank you". To place the word into the box you have made you will need to right click on the box and select "Format Auto Shape". The same box will pop up that was used to change the size and fill for the box. Click on the button that says "Layout". Now click "behind text". This will bring the square behind your type.
Now add a photo or something fun to spice up the front a little. I added a photograph of a fun texture in my example. You can be creative on this step. You can use a scanner to scan in textures or hand drawn art work. Whatever makes the postcard you.
The last step is to leave proper spacing for the back of the postcard. Specific areas of the postcard must be left clear for post office procedures. The picture below shows the specifications needed for the back of a post card. Now you have a personalized, homemade postcard that can be sent much cheaper than a card. All you need to do now is get it printed!