So the simplest approach if you just want to combine only the icons you want from existing free fonts is to use Fontello
The next step up with a similar approach is Fontastic. The big advantage here is the ability to add your own vectors.
Now if you want to go full out Inkscape has your back via a few quick built in steps:
- OBJECT → FILL AND STROKE
- OBJECT → ALIGN AND DISTRIBUTE
- TEXT → SVG FONT EDITOR
and there is of course a really great article to walk you through the whole process: