Solo instruments and sections
String samples come as solo instruments, violin, viola, cello and double bass, or as sections playing together. Each can be played with different techniques: sustained bowing (arco), short strokes (staccato or spiccato), plucked (pizzicato), tremolo and harmonics. A complete free orchestral library may include several of these for every instrument.
Building a believable part
Write as if for real players: violins on the melody, violas and cellos on harmony, basses doubling the cellos an octave down. Keep each line within the instrument’s range. Most free multisamples are recorded dry or with a little room, so add a hall reverb to the whole section rather than to each instrument.
Orchestral sample projects often release their work as CC0 or under permissive licences, which makes strings one of the safest categories for commercial use. The verdict on each card confirms it.