[dad911]

Those joints are big, you don't need a bag or cookie shooter.

Use a mason trowel, load the back side, then push it in with a tuck-pointing or gauging trowel. Mix on the stiff side to keep the stone clean.

https://marshalltown.com/cat-1328-gauging-trowels
https://marshalltown.com/cat-1253-tuck-pointers

edit: Re-read your first post. I've used a grout bag quite a bit. Mix needs to be on the soft side, well mixed, and I don't use the little metal funnel in the bag. Mix with a mixer on a heavy duty drill, in a 5 gallon bucket.

https://www.homedepot.com/p/ANVIL-Eggbeater-Mixer-57485/300960444