I always add a liner to the inside of my cards. This makes them sturdier and gives them a more finished look. I trim the liner a quarter inch smaller than the card base on all 4 sides (4 in by 5.25in)
Because a liner on this gate fold card is a little tricky to score and line up, I center the liner on the card base and mark the score lines on what will eventually be the back of my liner.
For the last step, I put tape on the back of the liner, fold in the flaps and put the center of the liner on top of the center of the card base.
Hope this all made sense.