Yeah, what Dave and Kristin said.
If you’re reducing your basal, do it one to two hours before you start exercising. It takes about that long (closer to two hours) for the insulin to peak in your system. I time my temp basal to end at the same time as my exercising ends. If, for some reason, you don’t actually get to exercise after you’ve set your temp basal, just stay on top of your BG and make it up with a correction bolus. It happens.
I usually eat before a work out and I start with an elevated blood sugar if possible, between 150 to 200 depending on what I’m going to do. I tend to eat a light sandwich and a piece of fruit - about 30 to 45 grams of carbs. If I bolus at all, I reduce it by 1 unit for each 30 minutes of planned exercise. Always carry a monitor with you (I recommend the One-Touch Mini) as well as fast acting carbs like candy, glucotabs or sports gels. For longer workouts, e.g. long runs of one hour or more, be prepared to carb up during your run.
I know, for instance, that if I’ve been running for 30 minutes and my BG is 112, I’m on the way down and I’m going to crash soon. I take three glucotabs (4g each - 30 BG points, for me). I try to test every 30 minutes during exercise.
Unlike Kristin, I don’t do a ‘make up’ bolus for the missing basal on the theory that the exercise has done the job of helping me absorb any excess glucose.
Trial and error, experiment, ymmv . . . yadda, yadda, yadda.
(Excellent advice on this topic is available in “The Diabetic Athlete” by Sheri Colberg and “Pumping Insulin” by John Walsh.)