About seam line marking !

I'd say the tutorial where JL goes deep into UV Unwrapping is this one:

https://cgcookie.com/courses/fundamentals-of-texturing-in-blender

But you are right on point, understanding things is the goal you want to pursuit, not only copying what you see on screen. So if you are jumping on Blender for the first time and you haven't watched CORE and the Blender Basics, you can also watch them here:

https://cgcookie.com/courses/blender-basics-an-introduction-to-blender-3d-4-5-lts

https://cgcookie.com/p/core

Hi ssinyeong11

Thanks you for taking the course.
UV unwrapping can be a complex topic. Omar already pointed to some great resources for an overall breakdown of the concept.
I like to think of it like unfolding a cardboard box. We need to be able to lay out the 3D piece in a flat way and place seams accordingly.

In our case, or in case of cabinetry and furniture modeling, we have to think about the real material we're working with. This refers back to lesson 6 "Fronts Overview". I'll try to break it down a bit clearer:

First of all we have to think about all the components that make up our piece, regardless of if it's a front or a shelf or something else. Here our fronts are comprised of 2 stiles, 2 rails and either a panel for the solid fronts, or muntins for the glass fronts.

With our UV seams we want to make sure we separate those components, so that the textures later reflect them accurately. So for example if we're working with stiles and rails we want to separate them with a seam.

Further if we're working with solid wood, we want to make sure we have a good flow of the texture. If we don't have a end-grain texture, we want to make sure that either the face grain (which I chose) or the edge grain of the piece flow nicely. So we leave the end connected to the face in one area.

Further for muntins and other pieces, which get overlapped, we have to follow the same principle. Think about how the pieces go together and place seams there.



Last thing we have to consider is what parts we actually see. Since in this case we're seeing the front, not the back, we have to be diligent with the seam placement on the front. We can then choose an edge on the back side of things to mark as seam, which ultimately allows us to unfold the UV island flat, without any stretching of distortion.

So to sum it up: The deciding factor for the seam placement in furniture and cabinetry is the intended material the piece is going to be build from and how everything goes together.
For different categories like game assets or others definitely check out the links Omar posted.

Hope this makes it clearer. Let me know if you have any additional questions.