app/views/drafts/_formatted_letter.html.erb
<% @content = @letter.content %>
<% current_replacement = @content.scan(/({{.+?}})/) %>
<% if @draft_variables %>
<% @draft_variables.each_with_index do |draft, i| %>
<% @content.sub!(current_replacement.flatten[i], draft.variable_value) %>
<% end %>
<% end %>
<%= @content.html_safe %>