Like Indy-Patti, I squirt a small puddle of Tombow Mono-Multi liquid in the green and white bottle on a piece of scratch paper, and then use a toothpick to apply small dots to the back of the letters or numbers.
I use a pair of tweezers to pick up the letter once the glue has been applied and carefully smooth it down.
The Zig two way glue pen is also good for this, and I use it if I have it handy (which means if I can find it, lol).
I don't think there is an easy way to apply small bits and pieces. It is just a tedious process and you have to take your time.