Sashiko thread it fat!
Even with the big eye sashiko needles, getting that very thick sashiko thread through the eye of the needle can seem rather like the proverbial camel :-D
Here is how I do it...
1. Make sure you have a clean fresh cut so there are no tiny fibers sticking off the end of the thread. I find it helps if it is a little damp and the end is flattened, so I bite it. My grade Eight Home Ec. teacher considered that an unacceptable habit and had a frown that made me flinch, but I'm too old for her to scare me now! :-D
2. This is a 'don't do this' step. Don't try to bring the thread to the needle and don't let the thread wave around in the air - so not like in this photo:
Step 2. Do this: Hold that fat thread firmly between your thumb and forefinger so only a tiny tip shows, like in this photo:
If you hold the thread firmly enough, its ability to split or fray is limited considerably so squeeze a little bit.
PS. Excuse my gardening hands :-)
Turn the sashiko needle so that its eye is positioned to go over the tip of the sashiko thread.
Notice how the sashiko thread is so firmly secured between my fingers that you can barely see it as the sashiko needle eye slides over it? It can take a try or two go get the eye to be positioned, and the needle to slide. Re cut the end of the thread with sharp scissors if you try this more than once before it works. Once that thread frays or splits, it gets pretty hopeless.
When the sashiko thread is through the needle, close your finger and thumb together over it so it is secured as you pull the needle further onto the sashiko thread.