1) take about a 6-8mm wide strip of KGD cotton and roll up one end to slip through the coil, should pull through with a little resistance but not so much you're tweaking the coil or ripping the cotton.
2) pull it so it's centered, and then push the cap back on the RBA, this leaves little indents on the ends of the cotton.
3) cut at the indents but at an angle so that on one side you're right on the line and the other a little past (leaving a little bit extra)
4) shove that little extra point into the juice channels, put the cap on, fluff the cotton at the juice wells a bit with something pointy.
5) prime it up and enjoy yourself.
That's what I do, or more did, and it worked fine. I didn't like the horizontal builds on the single RBA much, double is much better, and now I use vertical on the single. Just such long legs on the single builds.