Encoded Message

The method to encode the message is the pigpen cipher:

With that technique I encoded the message to Nildar (my encoded text is german, so if you want an english message you need to encode it first):

Decode Sheet

I kind of overcomplicated the decoding sheet. If you want to make an easier version, just draw the pigpen grid somewhere and exchange the letters for numbers, leave some numbers out if you want to make it harder.

Like this:

In the final sheet I color coded the lines to make it easier. Basically they have to fold the paper in the correct order to see where the characters go. The order is determined by the amount of strokes a line has:

1. First folds: 4 green lines, each made out of a single stroke
2. Second folds: 4 purple lines made out of two strokes each
3. Third folds: 4 red lines made out of three strokes each

Then they have to open up the paper again and fold the other direction:

1. Forth folds: 4 green lines made out of 4 stokes each
2. Fifth folds: 4 purple lines made out of 5 strokes each
3. Sixth folds: 4 red lines made out of 6 strokes each

The sixth folds are used to reveal the cross pattern

Example