Homemade Glucose Syrup is easy enough to prepare from scratch but doesn't keep more than maybe a month before it starts to crystalize. This is the reason I only make small batches at a time.
I use glucose syrup in all recipes that call for corn syrup, as we cannot get it readily here in the German part of Europe. It makes for quite an adequate substitute but does not add as much shine to royal icing as corn syrup does—both help with making royal icing a bit denser and easier to paint on, though.
Another use for it is in pastillage. And I found that painting a thin layer onto borders of surface-dried royal icing areas lessens and even prevents dark colors from bleeding into lighter colors.
Yeah, I'm not very patient when it comes to piping those details, LOL! So this has helped me to move on to what I love doing most a bit quicker!
200 g / 1 cup sugar
90 g / 3/8 cup water
1/8 teaspoon cream of tartar
1/4 teaspoon of strained lemon juice
1 pinch of salt
Prepare a cold water bath.
Add all ingredients into a heavy pan, mix with a whisk and bring to a light boil on medium high.
Reduce to low heat and simmer in covered pan for 10 minutes.
Uncover and cook on medium-high until the softball stage (234 °F / 113 °C).
Put the pan into the cold water bath to stop the cooking process.
Pour the glucose syrup into an airtight jar.
Store the syrup at room temperature.
You can half this recipe, if you really just need a small amount.
If you don't have a candy thermometer, then just wait until the mixture becomes syrupy like. You can tell that this happens when you put a spatula into the syrup and it runs off it in a slow string. As it cools down, the syrup will thicken some more.
Simmering the syrup for 10 minutes helps to make sure that all the sugar crystals are dissolved, otherwise they would clump together again too fast and you would notice that the glucose syrup starts forming crystals at the bottom of the jar within a couple of days already.
Here you can read more about the different stages of sugar syrup and what kind of thermometers to use.