First of all, you must make and attach a split lift, this is a piece of leather similar to the welt, which acts to lessen the natural curve on the bottom of the last. Heel building is a process of converting this curve to flat surface which can hit the floor and thus not unbalance the wearer.
The split lift is attached using a water based craft glue and nails. You place it onto the heel area, which you have previously glassed, to avoid squeaks. Then you bang in the nails till they just hit the last. If they go too deep you can't get the last out. Then you clip them off and punch in with a nail punch. This is so that you can skive the split lift flat in readiness for the first lift.Make sure that the surface is as flat as you can get it by looking along the top of it towards the front of the shoe.
Next you need to put some of the same water based craft glue onto the prepared surface. Be generous. The heel will be held together using glue and nails, so a stronger glue is not needed at this stage. Also, this kind of glue is easier to pull apart at the time of repairing the sole or heel and the repairer will thank you for not making their life difficult. 
Next you must cut out 2 heel lifts from some thick oak bark cow leather, similar to that used in the insole, only slightly less dense. Place the first lift onto the glue and secure it in the middle with 3 nails in a triangle shape. Punch them in as before. Look along the length of the shoe as before and skive off the surface to make it level. Use your knife or a ruler to check flatness. There should be about 1/8" gap at the heel breast. 
The next step is to put on more paste and place the second heel lift on and secure with 3 nails as before. Punch them! This time we are going to put in a row of nails around the edge which give the heel its strength. About 1/4 inch from the edge and spaced about 1/2 inch apart. Hammer them in and punch. I use a driver instead of a hammer. This is a big file and its teeth stop it slipping off the nail head and bending it. Again skive flat and check the levels with knife or ruler. When you bang the nails in, angle them into the centre of the heel about 10 degrees off vertical. This way, you won't reveal any nails when you rasp the heel. 
At this point you have to trim away all the excess leather with your knife. This must be kept very sharp or else you will cut and nick the upper. Also, the leather must be worked when it is "mellow". This means soaked for at least 1 hour and then dried to between 80 and 90% dryness. Trim until you have straight sides and a slightly pitched under back part of heel, following the curve of the last. 
Now you must use the French shape hammer to peen the edges. This closes any small gaps between the lifts and the split lift and creates a solid surface. Make sure the leather is well peened close to the seat because when you come to trim here, it is important that the seat ends up being tight into the upper. You can use the knife at this stage to take off any small lumps, making sure the sides are true.
With an ordinary biro, you now have to mark the heel lines at the end of the heel, lining them up with the silver pen marks you have put on the upper to mark the heel. I like to angle the heel line towards the front of the shoe, just think it looks better. Also, using a ruler or tape measure, mark the line of the seat, making sure it is even and level all the way round. Take your time here and get it right.
With a sharp knife trim off the seat line. It is absolutely essential not to cut the upper at this point. It is also really difficult to avoid it. Practice, practice! Wet the seat and trim it into the upper creating an even thickness of seat all the way round. Peen again as before. It is important to mention that you must keep checking the flatness and 1/8" gap at the breast with the second lift.
I will stop here and leave the top piece and finishing to the next post. See you next week.
