I use Microsoft Excel. To start you'll need an icon that has either a transparent background or a background in ONE colour (this is important). The colour of the icon itself doesn't matter. The ones that are black silhouettes are the best choice, but you can use stylized icons as well if you wish.
Anyway C+P (or DL & import) your image into Microsoft Excel
First you need to change the colour of your icon into the desired colour. Black icons provide the best results here (you can turn your icon into a black silhouette using some of the options available).
If your icon does not have a transparent background, you can change background colour to transparent here: I've highlighted the tab because I doubt your Dutch is as good as mine. You can also use the same option to change the colour of your icon if you dislike the colour options shown here:
The colours you pick from can be changed in this tab:
be aware that if you change a colour it WILL change the colour of ANY icon using that colour as well.
Anyway, the next step is to crop the image in a circle shape. If you icon is a perfect square, it will turn into a perfect circle. Adapt the sizes of your icon if needed to ensure height and width are equal.
Afterwards, fill in the background with a colour of your choice. I would recommend using a colour gradient between your chosen colour at the edges and a lighter shade in the middle.
Then you can play around if you want to. I usually add a shadow and call it a day. You can also add 3D effects which I've done below, which look cool on some icons and REALLY REALLY ugly on others.
Here's the final result, which is btw also my suggestion for Egypt