I have some very simple (I thought) goals:
1. Present two neighbouring 'H' glyphs with the stems closest to each other overlapping (and I don't care about next H(s))
2. Present two neighbouring 'L' glyphs with the heel of the right L just covering the toe of the left L (and I don't care about next L(s))
3. Present 'd' and 'P' with the stems closest to each other overlapping (and ...)

Common for all:
- basic Sans Serif
- the font will contain no other glyphs
- must correspond to latin characters/string (HH, LL, and dP) (not unicode 1000+)

I'm using FontForge, and I've tried to use kerning with no luck.
So far I've experienced the https://www.dafont.com/faq.php#nothing symptom, and tried all the suggestions in the FAQ.
- I have a GPOS lookup table of type 'Pair position (kerning)'
- with feature 'kern'
- subtable with the pairs and kern values (under the heading 'Δx #1', ~400 for HH and ~200 for LL)

Should this work?
What am I doing wrong?
Can I get the result I need in other ways?

In case you wonder, I could not be any greener with fonts, never worked with fonts until this week.
-AiwarikiaR

IMO what you need to do is create HH, LL and dP ligatures. That way you will get it exactly the way you want it.

feature dlig {
sub H H by H_H;
sub L L by L_L;
sub d P by d_P;
} dlig;

If you want it active all the time, add the statements to liga instead of dlig feature.

As for kerning, whether you like it or not, the kerning for HH pair will affect all instances of the pair in HHHHHH. The same is true for any other pair. Test the kerning in your font editor, which is FontForge, first before testing it in another program. Once you are satisfied with the result then test it in other programs.

I am not familiar with FontForge so I cannot help you on that but there are many here who regularly use FontForge.