There are 3 different types of grafting techniques you can use. The first is called cleft grafting. As you can see from the picture, you cut a V shaped cleft in the rootstock, and a point in the scion (the top bit, your heirloom or open pollinated variety), and then put them together using either grafting tape and/or grafting clips. This is not an easy grafting method. Remember the stems of both the root stock and the scion are just a few millimeters thick. It's very easy to mess up and not get a good graft with this method if you're not experienced at it. This method also requires a healing chamber, which while it's not rocket science is more work.

 The picture above shows side grafting. Side grafting has a higher success rate, but takes a little extra effort on the front end. You cut an upwards slice, 3/4 of the way through the rootstock stem, and a downwards slice 3/4 of the way through the scion (top part) stem. You then join the 2 together, using grafting tape or clips (they have to be fairly large clips, 2mm +) You are planting both seedlings together for a few days. You then wait for the graft to heal (about 5 days) and start cutting off the rootstock top, and the scion root, slowly, over 3 days. The benefits to this method is a higher degree of success, and no healing chamber required (though it's recommended) The down side is you have to mark which is the rootstock top, and the scion root, so you know which to cut. The worst result would be a rootstock top on a scion root. The exact opposite of what you're trying to achieve. I used this method this year. I tied thread around the rootstock top, and scion root that i wanted to cut off, so i could recognize the difference between the two. It's important, if you want to use this method to mark the 2 different parts. Once you seal the graft, the plants are wound around each other and it will be almost impossible to tell the top from the root unless you mark them well.

The most common form of grafting is called top grafting, or splice grafting. You cut a 45 degree angle in both stems and join them together. The benefits are, you don't have to worry about which is the rootstock and which is the scion (top part), the down side, not all grafts heal well, you need a healing chamber, and the success rate is lower than with side grafting.

Each grafting method has it's benefits and downsides. But if you take some extra time and a good amount of patience, you can graft your heirloom tomatoes to rootstocks that will give them higher productivity, higher disease resistance, and still get the best tasting tomatoes out there.