Kathy Spoering is a wonderful tapestry artist who lives in Colorado, USA. She has a blog at which she posts very inspirational and helpful information. Recently she wrote (and showed through photos) about her method of sewing slits. I've asked her if I might link to her post here and she was happy to let me do that. So... here's here Slit Sewing 101 post link. I'll also post this at the side margin so it will be easy to find.
Susan Martin Maffei also has a nice tutorial about several slit sewing methods shown at the ATA website at this link. That's noted in the side margin at links for interest.
And what method do I use? I usually use the Archie Brennan method that Susan describes. However, occasionally I still use an interlock for edges where I'm going to be carrying a color quite a long way in a vertical beside another, and where I want to see the "tooth" of the interlock.
All methods are equally good.