Customising an outfit in Nifskope for beginners
There are quite a few tutorials on this, but i haven't seen one that would list all this information in a form easy to use for a complete beginner. This assumes you can unpack .bsa's with FOMM, and find the files you need in your data folder.
You will only need Nifskope. Choose an outfit you would like to use as a base. Outfits that follow the body shape closely, ie Vault Suit, are usually best for this. We'll call it nif1. Open it in Nifskope. Go to Render / Settings, in a window that just popped up go to Rendering and click Auto detect game paths. This will display textures in the render window (only the ones in your data/textures folder, not those in .bsa's). Press f3, Block details window should appear on the bottom. Go to File / Auto Sanitize before Save, make sure it's checked. Choose an outfit with a part you would like to add to nif1. We'll call it nif2. Open it in another instance of Nifskope.
A few mods with plenty of cool 'loose' parts: The Pitt DLC, Tailor Maid, CRI Squad, Scavanger Outfits, Dragonskin Tactical Outfit, Military Equipment, Jill Valentine BSAA Outfit, FOOK2.
Combining your outfit
Keep in mind that there is no Undo in Nifskope. Save often and keep backups of various stages of your work. In the render window of nif2, right click on the part you want to add, Block / Copy branch. Back to nif1, in Block list (window on the left), right click on the 0 NiNode (top line), Block/Paste branch. New part is now added, displayed in a render window, and selected. In Block details (bottom window) of nif1, click on the small Txt icon in the top line. Change the name to arms (you can use numbers ie arms07) for everything that's on arms, this will make it appear in the 1st person. Use the name upperbody (again, you can use numbers) for everything else, this will make it disappear in the 1st person. Also, the part that is visible when your pipboy is on should be called pipboyon, the one that disappears when you put on your pipboy should be called pipboyoff.
It is safe to remove some part if you want (remember, most probably there is no body under it, just an empty space). Right click on that part, block/remove branch. Also remember, you can not undo that, so if you remove something you need you might have to start all over again.
Some complications you might encounter (you can skip to point 3 if you did not encounter any): If 0 NiNode (top line in Block list) of the 2 nifs you use has different names, you will get the message "Failed to map parent link NiNode <your nif2 0 NiNode name>". Simply rename 0 NiNode of nif2 to match that of nif1 (again, using the Txt icon in Block details top line), and try copying the part again. No need to save changes to nif2. Sometimes your nif1 will be missing a bone that nif2 requires. The message will be "failed to map parent link <bone name>". In Block list of nif2, find the needed bone, right click on it, Block/Copy branch. In Block list of nif1, right click on 0 NiNode, Block/Paste branch. Back to the nif2, the bone you just copied is still selected. Click the Txt icon in the block details top line and Ctrl-c the bone name. Back to nif1, the bone you just added is still selected. Click Txt icon, paste over the name (it is crucial that the bone is named exactly the same as it was in nif2). You should be able to copy the part you want now.
Adjusting the new shape
Usually the newly added part will clip through nif1 shapes. Right click on it, Transform / Edit. Here you can make small adjustments to its size and position. Avoid moving shapes much, as they are still connected to the skeleton, so if you move a shoulderpad to your knee it will fit when you're standing still, but when you move it will follow the moves of your shoulder, not your knee. If you want to scale the shape along X, Y, Z axes, Transform / Scale Vertices. Once you're done with editing, right click edited part, Transform / Apply. A message will appear, telling you that if you moved your shoulderpad to your knee it won't work ingame. Click OK.
Dismemberment (this part can be skipped)
If you paste, for example, complete arms from nif2 to the nif1 body, the meatcaps on nif1 (pieces of meat you see dangling from the wound when dismemberment occurs) will not fit the new arms. if on both outfits leg and arm meatcaps are separate, just delete nif1 arm meatcaps, and replace them with the ones from nif2 (using the method described in point 2). However, if arm and leg meatcaps form a single shape (that's usually the case), then there's nothing you can do without Blender. Wrong meatcaps will not damage your game, they'll just look fugly, that's all. Shouldn't matter much if you're making outfits for yourself and your companions.
Let's say you copied the combat armor chestplate from Tailor Maid to your new outfit, and you would like it to use one of many combat armor retextures. Copy the textures you need to your data/textures folder. In the render window, click on the chestplate to select it. In Block list, expand selected branch. Find BSShaderPPLightingProperty, expand it, find BSShaderTextureSet, select it. Now in Block details, expand Textures, you will see 2 texture paths - first one is the texture, and second one is the normalmap (it ends with "_n", it's there to make your object look more 3d). Click on a purple flower next to the texture and navigate to your texture of choice. Do the same for the normalmap if it's provided with your retexture, leave it alone otherwise (should still fit).
Hint - Steiner Combat Gear has great retextures of vanilla, Dragonskin, and Military Equipment meshes. FOOK2 also includes many great outfit retextures.
Go to Spells / Batch, click Update All Tangent Spaces. This will prevent many bugs. Go to Spells / Optimize, click Remove Bogus Nodes. This will remove all unused parts of the skeleton, useful if you're making an armor addon (Tailor Made style). Don't do this if you intend to add some parts later! Some tutorials advise stripifying TriShapes in Nifskope, I advise against it as this can distort a mesh (ie arms on Tobar's Outfit from Point Lookout DLC). This is best done in Blender. Save. You're done. :D