It is "strum a guitar" for playing a guitar.
"string something (tie attach to another or a fixing point) "
"string the boat to the mooring" although "tie" would be more common to a lay person
You can basically string anything that requires string or some sort of cord instruments fishing nets etc.
string would be used to describe making maintaining preparing anything that require string.
and "tie" would be the common word for joining attaching anything or anyone to anything or anywhere.