hi!! sorry for taking a while to answer, I wanted to make sure I could give you my best answer.
I usually do what's called a "double core" endband. I use double core endbands over the "bead on front" method because bead on front style is not great for uneven distributions of color, irregular patterns, or using more than three colors. Functionally it works by having your extra threads wrapped up inside the thread that is showing, forming the smaller secondary core. Ultimately you are doing figure 8s around the main core & then your secondary core of thread. This keeps things pretty neat & tidy. The tutorial I first used was this one by DAS Bookbinding, though I don't think his endband tutorials are his best ones. Another binder I've spoken with endbands about a lot is maleeka, who recently did an endband tutorial herself.
maybe I should do one... but it takes a lot for me to get enough motivation to make videos. I'll take this opportunity to write up some tips I've shared when people ask instead:
1. Endband core material is the MOST IMPORTANT component. You need a core that is stiff but flexible - it should NOT be floppy because it wiggles everywhere under the tension of the thread, but still needs to flex with the opening & closing of the book. You want something that doesn't compress, to reduce tension shifts in thread creating a lumpy endband. Have a smooth core is less critical but helps to avoid snagging threads & allows you some leeway on sliding threads around for adjustments. My personal choice is smooth leather jewelers cord (link is just an example, I get mine from a local craft store).
2. Thread size. All your threads need to be the same size; it will be visible if you are using two different sizes, and mess with your front core. Additionally, I know lots of people will use larger twists of multiple strands of embroidery thread, which can work, but is more likely to compress & alter its size in unexpected ways. A single strand is preferable. If you want something thicker you can find some thread weights that are heavier twists intended to be used in a single strand, not pulled apart. I prefer smaller sizes because it works better for the gradient designs I like.
3. Silk thread is your friend (if you can spend the money on it). It reduces fuzz (no fuzz like you get with cotton/DMC embroidery thread), it's usually easier to manage, has a more compact twist, and a higher shine. I use Japanese silk hand sewing thread in size #9 (9号). There's multiple brands (Tire, Daruma, KNK/kanagawa, etc). Here's a wholesale listing (minimum 20,000¥ for international). A non-Japanese brand is Guterman silk (German brand). Both the Japanese & German threads come in a heavier weight (Japanese is #16, Guterman is buttonhole).
4. Thread tension is the most important part of the actual technique. You need to ensure the threads currently wrapped in the secondary core keep tension when you are working the thread around them.
5. Working on a curve. This is only really relevant if you're doing an endband on a rounded book, but the circumference of the curve means there's more real estate on the outside vs inside of the curve. Sometimes this can cause bunching on the secondary core. My own solution to this is that sometimes I wrap the primary core but drop a wrap here or there around the secondary core (only between two wraps of the same color I'm dropping). I uh... don't know of anyone currently recommending this besides myself so I can't point to any pro endorsement for this method, it's just what works for me. Forgive my terrible writing:
6. Pattern management. I... don't really plan much how my patterns sit on the spine, which is not very helpful. HOWEVER you can do some pattern management on the fly, if you really want your pattern to end at a certain place. Thread can be packed more or less densely on the core, resulting in some pattern compression; you could also strategically drop wraps in less noticeable locations. An unintended example: I was replicating the pattern on this endband (left) when I realize I wasn't packing the thread as densely as I had the first time around (right), which resulted in the overall pattern taking up more space. You can do this on purpose, if you need to.
this was way more than you asked but it gave me a chance to put all this in one spot. Best of luck in vanquishing the dreaded EndWyrms.
