To continue in plain stitch just set your cam lever back to stockinette and push both the side levers on the carriage towards you (that's what I do). You can leave the card in or you can take it out, it shouldn't make a difference.
To continue in another pattern immediately after the first pattern you need to change the card, lock it on row 1. Release the handle of the carriage and lift it up so it's tilting backover, remove the yarn from the carriage, move the carriage across the needlebed and back again to memorise the punchcard pattern then put the handle back down, set the carriage to the required stitch pattern with the side levers back, replace your yarn, unlock the punchcard and carry on knitting.
If your starting a pattern after plain stockingette you can put the card in any time that you're knitting the stockingette and lock it on row 1 until you're ready to start the pattern.