The easiest way, in my opinion, is to get an external SSD drive, and plug it into a USB 3.0 port (they're the ones with a bit of blue in them, I believe.) But it's not a particularly affordable solution, starting around $100 for the smaller varieties on Amazon.
If it's an older game (which it might be if it can run on older specs), you might be able to get by with a cheaper external HDD , provided it's USB 3.0 compatible (otherwise it'll run too slow.) I have a USB HDD that I mainly use for storage, but I can also run older games on it with no problems ("older" in this context being "before 2012.") I paid $60 for it and it's 2TB.