The jHeretic Resource Pack is a pack of High-Resolution textures, MD2 model replacements for in-game 2D sprites and various visual effects made for jHeretic (Doomsday Engine). This pack is recommended for everyone because of its high quality.
Locate the installed Doomsday engine folder (most probably in 'C:\Program Files\Doomsday')
Open 'snowberry\addons\' subfolder.
Unpack the 'jhrp1_01.box' folder from the JHRP archive to the 'addons', so the resulting directory structure will be something like this, depending on the location of Doomsday folder in your system:
Start Doomsday, select 'Shared Settings'->'Addons' and check the checkbox near 'jHeretic Resource Pack'.
Select the game type ('Heretic' for example) and click 'Play' to start the game.