How do you use a macro to prepend text from a register to each line?
qaI<C-r>a<Esc>jq
Record a macro that inserts the contents of register a at the start of each line using a in insert mode, then moves down.
Tag: register
How do you edit a previously recorded macro in Vim?
:let @a='<C-r>a'
Paste the macro register contents with :let @a=' followed by a to insert the current contents, edit them, and close the quote.