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:

How to Make Your Own Icon Webfont