On average they should hit between 1/2 and 2/3 of the time.
Their hits should be sized so that if the entire encounter ganged up on one PC they could do damage on par with that (one) PC's full HP in one round.
As for saves, if it's a save the monster's proficient in make the save bonus on par with the caster's "DC bonus" (which effectively makes it a coin toss as to whether the monster saves); if not proficient make the save bonus one or two.
Give the monsters HP such that if the full party hit for a round with their average/default attacks, it'd eat about 1/3 of the monster(-party)'s total…