Fontself Maker includes font family support, each font style being handled as a separate font file (it does not yet support multiple font styles into a single font file, like the TrueType Collection .TTC) format.
Create a font family
When you design several versions of your typeface (like Bold, Italic or Fantastic), it is highly recommended to layout all the glyphs on a single document (to ensure all glyphs have a relative size) and to use one or several guides (for precise alignment).
Here is how you should proceed:
Create a first font based on your default style and save it as a font file, i.e. MyCoolFont.otf
By default, any new font's style name is called Regular, so if you want to call it differently, go to the Font Infos menu and change the style name from Regular to Other... and type whatever new name you want.
Once you are ready to create an additional font style, click on New > New Style and pick a new style name. This will create a new empty font that has the same font family name and which includes the same vertical metrics as the first font.
Drag the shapes of your new style (ideally with a baseline guide) and save it as a second font file, i.e. MyCoolFont-Bold.otf
To add a third style, press again New > New Style and keep going on.
Matching spacing or kerning from other font styles
To speed up your workflow and ensure font styles share similar features, you can import spacing & kerning values from another font file.
Matching glyph size across different fonts
You can also precisely change the scale of all glyphs according to a specific value by entering a capHeight or xHeight value.
Mind that capital glyphs are required in your font when you change the capHeight (and alternatively, lowercase glyphs are required to scale when editing the xHeight value) - otherwise, the option to Update & Scale Font will not be displayed.