If the split is extensive {I presume it's along a seam?}, best option is to take them out and do them properly. If you have a sewing machine, that would be better than by hand. It might be worth giving them to a professional to resew all of the seams for a longer life since, once you sew up one split seam, another may go.
The seats come apart at the base/back joint. Once apart, the back unzips at the bottom and is then hooked through the back at intervals all the way up. Care and attention as you work. The headrest is held in place by a screw which you can only access with the cover pulled up. This can be baffling if you don't know about it.
The base cover is held on by clips at front and rear and retaining wires at the sides. It is also held in the middle by clips coming through the cushion to be spring base. Check out where they are first, unhook and take care undoing stuff.