I make my own harnesses frequently...nothing tricky to it. you have a choice of where to tap into the stock harness for power, my recommendation would be to tap into the power supply where the horn is powered from (it's a black wire) the main harness.
add a flasher in line between the power wire (black) and whatever handlebar switch you want to use.
from the handlebar switch run a left (orange) and right (light blue) signal wire (split each signal wire into two and run one to the front and one to the back turn signal). then you can ground the turn signals directly to the frame or tap into the common ground from the stock harness (dark green wire) anywhere that is convenient.
since the charging sysem on the SL is even weaker than the CB and CL's I'd recommend against adding dual element turn signals with always on running lights. I'd also recommend using LED turn signals since they drawl very lilttle electricity