Here’s a chord progression which modulates:

C-7 A-7 F#-7 Eb-7

Decide on a scale, let’s say Dorian:

C Dorian: C D Eb F G A Bb C
A Dorian: A B C D E F# G A
F# Dorian: F# G# A B C# D# E F#
Eb Dorian: Eb F Gb Ab Bb C Db Eb

In the picture the notes are arranged so you can see the similarities and differences in the scales.

Find a line that goes through the chord progression.

Play it using some regular note value and try to keep the line moving as you repeat the four bar chord progression.