Quill or cotton swabs (Q-Tips)
Heat source (toaster, stove element, embossing gun, hair dryer, etc.)
The last step of the instructions should never be attempted by kids without parent surveillance. Also, handle the paper carefully as it could catch on fire.
Dip the quill or cotton swab in lemon juice and write your message on your piece of Muggle paper.
Note: Experiments with invisible ink on parchment paper led us to believe it doesn't work. The parchment just catches fire without revealing the invisible ink. A spell against Muggles the wizards have put on it, probably.
Make sure to dip your quill or cotton swab often in the lemon juice so you have enough invisible ink on it at all times.
To reveal your message, place your parchment or Muggle paper over a heat source and watch the message appear on your page. Almost like real magic!
© MarcT's tutorial was taken with permission.