I just did one of these last night. Let me see if I can remember the measurements off the top of my head. Cut a piece of cardstock about 5 3/8" by 8 1/2". Score lengthwise at 6 3/4" and also at 3 1/8". These are your folded flaps. the slot punch will be used on the larger flap. Put your slot punch all the way in on both sides and punch kind of in the middle. I also did a 3rd punch on the botton to hold the gift card more securely. (I noticed when punching with these measurements, the slots barely held the gift card in. It was adequate, but if you wanted a more snug fit, you may want to change your beginning measurements to about 5 1/8" by 8 1/2". You might have to play.) Next, Make a piece that measures 4 1/4" by 2 3/4". This is the biggest part. If you want to have a mat, make your other pieces smaller accordingly to mat. This will go on your shorter flap, and will cover the slit marks you made on the wider flap once it's been folded.
I hope that explanation makes sense. I'm not real great at giving directions.