Hiccup Horrendous Haddock III uses his journal to pen down information on the go. He also uses it to draw the outlines of his gadgets, inventions and new dragon infromation. He has been seen sketching dragons and maps with his journal. Hiccup kept it in his vest in the first film and flightsuit in the second film.
In How to Train Your Dragon, Hiccup draws a map of Berk's forest in his journal while trying to find the downed Toothless. He also draws a drawing of Toothless in his journal to see how the new prosthetic tail fin will look once added onto Toothless.
In the TV series, Hiccup uses the journal to copy information from another source. Particularly in We Are Family, Part 2, he copies down a map to the Isle of Night, and the other teens are able to track him down and rescue him from Alvin's clutches because Astrid found his journal with the map still in it.
In How to Train Your Dragon 2, Hiccup still uses his journal. However, its purpose has changed; it instead holds his folded map. He keeps it in the vest of his flightsuit and uses it to record and map down other lands. He adds on more pieces of paper by having Toothless lick it and cover it with his adhesive saliva.