A good first start for creating an oblique is the Transform filter, one of the pre-installed filters that come with the standard installation of Glyphs. Your mileage may vary, but good oblique angles are usually somewhere between 6 and 12 degrees.
Choose Filter > Transformations > Transform, and do not translate (i.e., leave the values at zero), do not scale (leave at 100%). But do pick an Origin: As best practice, we recommend picking half x-height because this is also what Glyphs uses for calculating italic sidebearings. That means that you can use metrics keys with cross references between LSBs and RSBs. Don’t know what that means? Take a peek at the Spacing tutorial.
Slanting vs. Cursifying
Then pick an angle and a mode of knocking your design into an italic angle. The Transform filter offers two possibilities: The first option, Slant is a simple geometric shear as you probably know it from your favorite vector application. The problem is that you will lose the weight of verticals through the distortion while your horizontals stay keep their weight. Not good.
The second option, Cursify (click on the word Slant to switch to Cursify), tries to counteract the distortion of curves. Here is a comparison:
But how does Cursify know what to do? It simply makes use of your standard stems for calculating curve corrections. So do make sure you enter appropriate, average horizontal and vertical stem values in File > Font Info > Masters (Cmd-I):
Pro Tip: Since the standard stems entered in File > Font Info > Masters determine the curve correction, you can influence the amount of correction by experimenting with different stem values. Or, even better, put the cursified version in the foreground, and a merely slanted version in the background. Then interpolate between the two by using the slider in Filter > Transform > Background:
This can be especially useful when the counter appears overcorrected and too pointy.
Fixing Extremum Points
Also, the positions of your path extrema on curves are not adjusted accordingly. This makes vertical hints impossible and some operations such as interpolation or nudging pretty difficult. If you do not care about hinting, and you are not interpolating, or relying on nudging, well, you do not need to care about your extremes. You can keep your paths like they are in this sample on the left:
However, if you need to hint your vertical stems, and plan to interpolate, it is a good idea to fix the extremes, like in this sample on the right. You can use Paths > Add Extremes, or Shift-click a curve segment with your Draw tool (P) to insert a node at the nearest extremum. To get rid of the slanted former extremums, select them one by one, and press the Delete key. Glyphs will do its best to reconstruct the curve segment as well as possible, but you may still need to adjust curvatures here and there with the Fit Curve palette (Cmd-Opt-P).
Toshi Omagari wrote a script called Path > Delete Diagonal Nodes Between Extremes that speeds up the process by trying to remove all those diagonal extremums, on all masters, at once. You can install this script, along with lots of others, by installing Toshi’s Scripts repository from GitHub. Follow the installation instructions in his readme (scroll down a little).
Correcting Straight Stems
But be careful: While Cursify can do a pretty good job with your bowls, straight stems will still simply be slanted. But the higher your angle, the more extreme your slanting distortion, i.e., the more your vertical and right-leaning stems will thin out, and the more your left-leaning stems will bolden. So be prepared to adjust your stems a little bit afterwards. For example, slanting a vertical stem 30 degrees will thin it out by approximately 13 percent. But if the stem used to be 30 degrees left-slanted, and now is slanted into the upright, its stem weight will gain no less than 16 percent:
Needless to say, 30 degrees would be a very extreme angle. I chose it for this example to make the stem distortion more visible.
Update 2015-07-30: Updated screenshot for Glyphs 2.
Update 2016-12-03: Partial rewrite. Updated and added screenshots, added slant distortion, extremes, titles and illustrations.
Update 2016-12-05: Added Pro Tip, moved note about slanting origin to where it belongs.