Metrics & Kerning for bespoke ligatures

Hi there,
I’m creating a handwritten font but having trouble with the metrics/kerning for a series of ligatures.
The default ‘fi’ and ‘fl’ work fine but I cannot work out how to make bespoke ligatures behave in the same way.

A direct email to High-Logic support came back with this:
First ensure the left and right side-bearings are correct, then add kerning to them.

Hope someone can help with this.
Cheers,
Dave

For example, to create an fl ligature, the left side-bearing would be the same as the left side-bearing of the f; and the right side-bearing of the ligature would be the same as the right side-bearing of the l. The l glyph in the ligature would be advanced by the advance width of the f.
fl ligature.png

Thank you so much Bhikkhu this has been a great help.
Cheers,
Dave