How can i stretch my dirt bike tire onto the rim?

im putting a dirt bike 12in tire on my 12in dirt bike rim on my 1998 ktm 65sx and i cant seem to get the tire the rest of the way on, theres like 5in left. its an older tire that has been setting around in the garage for about a year or so because i never needed it. can i put something on it to make the tire more soft and plyable so i can stretch it or should i just take it to a tire shop where they have fancy tools to put it on?