A huge part of siliconing is the initial application, don't apply a stinking great bead if you only want 6mm, it'll end up everywhere, just cut a small taper in the end of the nozzle. I've been using profiling tools for around 5 years now and would be lost without them but, I've always struggled to get a nice clean join where 3 lines meet. I find that it's best (for me any way!) to do all the verticals first and let them set up for half an hour or so then do the horizontals, a lollipop stick is very handy for getting right into the corners as well. Soapy spray has never really worked for me so it's not something I do, just get that initial application right, and whichever way you go from there will leave a great result with practice. Try a few things out to see what works for you.