For disarm, roll to-hit as normal. If you hit, they get a saving throw. If they fail that saving throw, they're disarmed.
That's assuming that being disarmed is a pretty big deal (so something like: pulling out a new weapon or recovering their old weapon takes them a round and invites an attack).
For a trip, maybe something like... make a to-hit roll as normal. Then they get a saving throw. If they fail, they're on the ground and you can make a follow-up attack (with whatever bonus you award people for striking somebody on the ground: maybe +2 to-hit). They must spend a round in order to get to their feet.
Note that both of these maneuvers have a fairly high bar, because you have to hit and they have to fail their saving throw. Thus, they may work best if you already have your enemy at something of a disadvantage. If you're receiving a +2 bonus vs. them for some reason, for instance, this bonus could be applied to both your chance to hit, and subtracted from their chance to save. This way, there's less incentive to just go around trying to trip people over and over. (Though I'd start giving them penalties for being obvious if they tried to do that in any case.)
If putting your opponent at a disadvantage isn't something that happens naturally with the way you do combat, then you could let people do a maneuver to set it up. Basically, you get -2 to-hit to one attack in order to set up your next one (getting +2 to that one). Or, if you want to make it trickier, you do -2 damage if you hit, but any hit roll (regardless of if you hurt your target) means you get the +2 to your next attack.