open blockbench and create new java block/item and name it the item you want to replace
get an image of your terraria weapon(preferably the lowest possible resolution so each pixel in the weapon is a pixel in the image)
click import texture on the bottom left and select your image
open the image in another image editing program to reference as you work
now click create cube on the right side and resize to 1x1
create one of these voxels for each pixel in the weapon, moving them into their corresponding places.
now you have to recolor each voxel with the correct color, selected from the texture(image) you have imported.
to do this, first click on the "16 x 16" in the top left corner, and enter the resolution of your image.
then, selecting each voxel, right click on the highlighted cube entry in the list of cubes you have created on the right.
on the drop down menu, click texture, and then the image you have selected.
the image will appear on the top left. along with a box that shows the currently selected portion of the image that will be applied as a texture to the selected cube/voxel.
you must resize this rectangle to 1x1 and drag it to the pixel on the weapon that corresponds to the voxel on the model you have selected.
now click apply to all faces, which is an icon in the row of icons right below the image.
do this for all the voxels and pixels.
now you have a model. click display in the top right of the ui and adjust it how you like. it will appear in game the same as how you adjust it here. Then export your completed model as a json.
copy the file structure I have for this pack and replace your json and image where I have mine. make sure they are renamed correctly
you are done now.
good luck

Sorry, this won't work for pocket edition, if that's what you mean.

Thank you! I am not sure about a 1.8 version though, those usually take a while; maybe I'll get around to it in the future.