The figure 5/4 should be treated like a 4-3 suspension. Of the two numbers in this figure, the 4th is the suspended dissonant note, and the 5th is the 5th of the root position triad. (Most of the time the 5th should be included in a normal 4-3 suspension anyway). The 4th should resolve down to the third, even if the 3rd isn’t specifically figured.
In this example from Corelli’s Trio Sonata op.2/7, the 5/4 figure requires a 4th and 5th above the bass note A. The 4th (D) is the suspension note, and thus needs to be both prepared in the previous chord and resolved in the following chord, in the usual manner. The 5th (E) is the 5th of the Am root position chord.
Sometimes there is a decorative figure between the suspended 4th and its resolution, and this can make the suspension figure more difficult to spot.
In this extract from Corelli’s Trio Sonata Op.1/1, there are two chords between the suspended 4th and its resolution (the prevailing key is C major): The 5/4 figure requires the notes C (4th) and D (5th) above the bass note G.
The C is the suspension note, and needs to resolve down to B, but before this happens an auxiliary chord of IV7 (Fmaj7) is inserted, returning to a root position dominant with suspended 4th, finally resolving at the end of the bar. The C is held until it can resolve.
This progression (V4-IV7-V4-3) is relatively common in the Baroque style at a cadence. Here is another example, which is decorated in a slightly different way, from Corelli’s Trio Sonata Op.1/4. The prevailing key is E minor.