Also known as Tic-Tac-Toe Ciphers and Masonic Ciphers.
The alphabet is written out in two Noughts and Crosses grids, using dots for J to R, then in two diagonal crosses, using dots for W to Z, as shown.
To encode a message, use the grid pattern with dots as appropriate.
But this method is very well known, so just use it for fun.
Here’s one for you to solve: