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 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.