When we're improvising over something which is modal, it usually means that the harmonic backdrop in the song we're improvising over doesn't change too much or at all over time.
An example of this could be that we're playing along with a backing track which is in 1* minor, and doesn't change chords at all or minimally. If we'd like to improvise with this backing track we'll need to understand what ai's this backing track is defining and also have the ability to know which ai's we are playing at any moment in time.
If you've read through "Scales on Guitar", then you might have a good idea of how to proceed with this, nevertheless we will examine how one might do so.
We'll first start by locating one of the 1*'s on the guitar, which you should know since we've gone over how to locate any note on the guitar. Once you have played this note, you'll want to then remember which ai this represents. Since we've set our anchor note to be 1* (the root of the key we are playing over, a common choice) then it's clear that the interval between them should be 0, thus our current notes ai is 0.
From this point in time onward, as long as we keep track of the current ai we are playing then we will be able to hear the effects of playing different ai's against the backing track. If we only play ai's the the minor scale is made from we'll notice that what we play seems to match what is in the background and isn't an outside sound. Simply being able to play ai's which match the backing track's might not be as simple as it seems as a beginner, so for reference you should go back to the article about fretboard movement so that you can see how different movement changes your current ai.