Your best "base" for suet cakes is actually beef or mutton tallow. If you have a butcher nearby, you may be able to get it very inexpensively. Lard works as well, but it tends to melt at a lower temperature, so if your feeders are out in the sun, it can easily turn into a real mess.
The rendering process is basically cooking down pork fat (lard) and beef suet (tallow) until the all the fat has been melted away from the gristle or tissue, and the fat becomes liquid.
Rendering lard or tallow is pretty straightforward. Take the hunks of fat, and cut it into small pieces. Then put it into a pot with about a quarter inch of water in it. Heat the pot very slowly, until the fat is liquid. Many people use a crock pot to do this.
Once the fat is liquid, you'll need to strain it; I'd suggest cheesecloth (which you can get at fabric stores), or a metal strainer. Be very careful; the fat will be hot, and burns can be nasty, of course! I suggest using canning jars (which can still be bought at most grocery stores). Strain the clear fat into the jars, and the put the lid and ring on the jars and allow them to cool. Properly canned lard will last almost indefinitely.
You can mix berries or nuts into the fat before you can it if you're going to use it strictly to feed wild birds; otherwise, you can simply melt the fat down again as you need it, stir in whatever ingredients you want to use, and let it harden again. Lots of birds like the fat with peanuts added; I sometimes add "store-brand" peanut butter (don't add a lot - it lowers the melting point quite a bit). You can feed it from mesh bags or from suet cages. If you live in a warm area, you may want to add a bit of corn meal to the fat to keep it together better.