Roof mortaring

I'm learning how to accurately drop mortar into the joints. I often miss and then the cement makes stains on the bricks. I also make a lot of crumbs and I have to resist the urge to brush them until they are dry, because otherwise they also stain the bricks. The best way, I've found, is to be patient- only take a bit of mortar at a time, and prepare the little bit on the big trowel before using the little trowel to flick it off. I have to make sure the joints are fully filled but I also don't want it to get super messy. I still have a lot to improve...