When creating a texture for your 3D object, photoshop might be your biggest help and this blog post will go over the basics of how to create seamless tiles.
#1 Image size / resolution.
Simple but important, the size of a texture is almost always ^2 (to the power of 2). Whilst I’m not knowledgeable enough to explain the in-depth reasoning, I understand that textured are displayed through ^2 because it’s on a binary system. Whilst making the resolution any number is possible (and probably slightly more lenient on current machines), the texture will suffer due to scaling up to the correct size.
I personally used 1024×1024 pixels for my textures.
#2# Offsetting.
You have your texture set to a ^2 resolution. Your next step is to offset set it through the filter tools (marked under the “other” tab). Depending on where you want the seams to be and the texture to offset you could decide any pixel on the x+y axis to be the crossover point, however I went with 512×512 as it was central and guaranteed even tiling when the texture was applied.
#3 Removing seams
The time consuming part. Removing seams can be done by using and array of tools such as: Spot Healing Brush, Patch Tool.
-Spot healing brush
A very basic tool that is incredibly simple and easy to use, by clicking on an area, photoshop will calculate what should be in its place by using the surrounding colours. It’s effective for removing small patches that seem unnatural however due to the tool calculating the colours it won’t be 100% accurate because it’s not a replacement for detail.
-Patch tool
The patch tool is a bit better for larger areas, or places on your texture that stand out too much. By drawing over an area, said selected area will change depending on the mouse placement. Copying other parts of the texture in its place. Similar to the spot healing brush, it’s helpful for removing bits of unnatural or unsightly areas. It’ll take a bit more time to use if you’re using it to rectify unnecessary detail.
Jonathan's University Blog 