Here is a very simple recipe found on Zaar
To Preserve Goose Lard: German Ganseschmalz
There are much more recipes in Germany. Often they are combined with lard from prok which contains the "Flomen". If you want to make goose lard you should buy lard from pork for it. Otherwise you have to make it in advance.
Goose lard is typically made as side product of the traditional roast goose for for St. Martin's day (11 November) or christmas (25 December).
You need the fat from one goose. You could refer to the recipe above.
1 part goose lard
2 parts lard (from pork)
about 5 onions, chopped
about 5 apples (variety Boskop, if available, otherwise a good baking apple variety, little tart), chopped
salt to taste
mugwort (grounded)
majoram
Heat up slowly the two lards. Stir as required. As soon as the "Flomen" (the little fluff balls in the lard) are turning brown add first the onions and a little later the apples. Add salt, mugwort and majoram to taste. Stir as often as required. When the onions and apples are golden brown remove from heat and fill into suited dishes. After it is completely cooled down seal the dishes and store in the fridge or in a cool place.
You can use it as a spread or to season red cabbage recipes with it. You can also use it to fry potatoes.
The reason for mixing the goose lard with lard from pork is that goose lard is very soften. If you like it very soften you can reduce the amount of lard from pork.
The amounts of onions and apples are only a guess. If you don't like the fruity note use less apples. If you like the onion note you can add some more. You just have to try what you like best. Every family has an own "tradition" on how to make it. My mother don't use any lard from pork at all.