Minecraft wiki entry for Creating_a_resource_pack, associated text: On the menu bar, go to "Edit -> Add Cube" to create a new block.
Resize the model to a full-sized cube. Then, under Textures on the left panel right click ladder.png, then click Apply to Untextured Faces.
In the top left, there will be an image of the ladder, with a 1×1 highlighted region. Move this to change the texture to what you want it to be. Repeat until all sides of the block are as wanted.
Go to "File -> Save Model" and save in resourcepacks/Tutorial_Resource_Pack/assets/minecraft/models/block/ladder.json (Replace "block" with "blocks" for 1.12 and below).