When I yank some text into a register, how do I append the text to the current contents of the register?

When you specify the register for some operation, if you use the upper-case for the register name, then the new text will be appended to the existing contents. For example, if you have some text in the register “a”. If you want to append some new text to this, you have to use the “A” register name. If you use the lowercase register name, then the contents of the register will be overwritten with the new text.