Trap tutorial

In the following video Sundog is used to create a Trap song. The tutorial focuses on chords and melodies.

Key points from the video

Use the minor scale for chords and melodies.
Work with triads, not four note chords.
Chord notes should be close to each other (open chords are too bright).
Minor chords and sus chords are nice.
Don't change chords / bass notes too much.
Play the 808 like an instrument.
A note interval of one semitone feels dark/dissonant (and is great for melodies).
Create small melodic variations.

Some thoughts on sound design

This tutorial concentrates on chords and melodies, because there are already quite a few tutorials available on YouTube that will tell you more about Trap sound design, mixing, and instrument choices.
But here are some key concepts:

Tune your 808 bassdrums! Many 808s are played back at F, F#, G, or G#. Use your sampler to tune them to C. This way you can play them like other melodic instruments.
If your kick is not punchy enough, try to set the 808 volume lower than your kick volume. If the 808 is too loud, it can dominate the mix and make the kick feel weak.
Brass sounds are perfect to create an aggressive, threatening atmosphere.
Bell sounds are typical as well. So are sirens, rising synth sounds, gun shots, and short vocal shouts ("chants").
Chants and other vocals are often pitched to a lower note than the original sound.
Your snares should be short and snappy.
In many cases chord instruments can be made stronger by playing the lowest note of the chord (bass note) one octave below as well.

Sundog Song Studio: Sundog is used in this video to create the different chords and melodies.
You can download it here: http://feelyoursound.com/sundog/ (Windows, macOS)
A quickstart tutorial is available at http://feelyoursound.com/sundog-quickstart/ (7 minutes and you will know everything you need)

BPM and swing: Most Trap tracks will play in the range of 130 to 170 BPM. However, they "feel" like 65 to 85 BPM. The faster BPM setting is used to better work with fast hihat and snare rolls. Many (older) drum machines only work with sixteenth notes, so a 140 BPM hihat roll will feel like a 70 BPM thirtyseconds roll.
Swing is only rarely used. A certain swing feel can be created by delaying the hihats by a few milliseconds.

The scale: The minor scale is the dominant scale for Trap songs. It generally feels deeper/darker than the major scale.

Chords: The most interesting chords for Trap can be found among the commonly used triads of the minor scale. Four note chords are only rarely used, because most of the time they sound too smooth and jazzy.
You should prefer minor chords over major chords as they feel much darker. The notes of your chords should be very close to each other, because if they cover a larger range of the keyboard they feel too bright and open.
Suspended chords are very interesting as well. Their ambivalent nature between major and minor chords adds to the gloomy atmosphere.
Regarding chord progressions: Don't do too many chord changes. If you change chords too often, it will be hard to maintain a threatening atmosphere.

Melodies: Some notes of the minor scale are more interesting than others. Concentrate on those notes that are only one semitone apart from each other and make sure to play them in succession. The semitone interval sounds quite dissonant, so these melodies will strengthen the haunting mood.
Many melodies use a very simple rhythm and are rather short. Take a small arpeggio for example. Play it back three times and do a small variation at the fourth repetition. This will add some diversity but will keep the basic atmosphere intact. Another trick to create some variation is to transpose the melody one octave up from time to time.