You can find some good advice about this in the Typophile archives.

Here's what I myself recommend:
1) Make a midling weight that looks good for text.
2) Make a Bold "candidate" (maybe using auto-weight-gain + mucho tweaking).
3) Extrapolate the lightest and darkest extremes.
4) Clean up the extremes real good.
5) Make new intermediates and compare against the originals.
6) Tweak only the extremes to make any new intermediates come out good, dumping the original intermediates.

In this way you have clean results but you never have to tweak intermediates again.

hhp

The easiest way, I think, would be to make the extremes, like light and black, make a MM axis out of this, pick the intermediate weights you want, and tweak them up. However it

This is how I do it:
1) Prepare an ultralight (maybe even stroke weight 0) monochrome weight.
2) Extrapolate the extrabold. This will be too "proportional", i.e. the stroke weight is constant, makeig a and e too bold.
3) Modify and clean up the extrabold.
4) Re-interpolate the regular wight.
5) Create an MM font with the "true" regular and the interpolated regular wight.
6) Extrapolate a "triple true" regular (truer that true!). Now you can see what makes the true regular true and make conscious decisions about that. I think the most important thing with designing is that you do everything consciously.
7) clean up the triple true and re-interpolate the true regular.

When I generate the final instances I put all three weights in one MM font. The semibold is not only an interpolation between regular and bold but I add a certain amount of extrapolation (regular and light).
The resaon for this is that a point does not move on a straight line on its way from light to bold (that is why we need a true regular). However, it should not move on a line with a kink at the regular weight. It should move on a curve instead. So the semibold will not be on the direct line between regular and bold.
I usually prepare an Excel diagram that shows the amounts of light, regular and bold in all the instances. Again, the graphs schould not have kinks. The semibold usually has about -10% of the light weight, 60% regular and 50% extrabold.

Yours,
Tim

> Create an MM font with the "true" regular and the interpolated regular wight.

An interesting idea.

> I think the most important thing with designing is that you do everything consciously.

I agree, although I've realized there's a certain ideological paradox here: because we don't fully understand readability, there has to be a place for uncertainty, somehow. This is why for example I see something wrong with constructivism, which is present for example even in the otherwise supernatural FF Legato. But I guess that's another thread. :-)

> a point does not move on a straight line

BTW, is there now a way to implement non-linear MM interplolation (outside of working at Adobe :-)?

hhp

>is there now a way to implement non-linear MM interplolation (outside of working at Adobe :-)?

Check out Letterror's SuperPolation
http://www.letterror.com/code/superpolator/

As shown above, there are many ways to go about interpolating.
My best advice is to try as many as you can and compare the results.

Maybe really obvious to some but (I think) a good trick is to create
one interp for the caps and one for the lowercase and then combine
them. This way you avoid the somewhat anemic interps that result
when using light and black masters. I do this all the time. I even
make a third sometimes for the numbers and a fourth for the fractions
and punctuation. Of course, then the hand tuning begins.