Printer for applying royal icing

The ingredients that make icing creamy and give it the bulk for piped decorations, tend to be items that will go softer in heat (butter, shortening, egg whites, whipped cream). A glaze might set up if you made it sufficiently thick, but it wouldn't allow you to do anything like pipe borders.

While I know it's not really a solution to what you want, your best options are probably to decorate with non-icing items like fruit or candy. Probably the closest imitation would be to try using marzipan, which can be molded and coloured and I think would probably hold its shape better under heat.

Buttercream is pretty simple, start with room temperature butter, beat it until it's white, slowly add icing sugar, add small amounts of milk as needed to keep the consistency right.

For each 1/4 cup (1/2 stick) of butter, use about 1 1/2 cups of icing sugar (sifted) and 3 Tbsp of milk. You can do it in a mixer, a food processor, or just use a bowl and a whisk. I used a whisk a few days ago when I realized my large stand mixer is overkill for the small amount of icing I wanted and I found that microwaving the icing for about 10 seconds helped me stir in the sugar when it was getting too thick.