Hello Olivia,
There's quite a few things to consider with exercise starting with the type of exercise, as the type will impact your BG levels differently depending on the impact, if it's high impact aerobics or running at pace then you are likely to release cortisol which will elevate your levels, low impact does the opposite, time of day has a part to play as does insulin on board, weather or temperature, and hypos within the previous 12 hours. There's alot to consider..
My golden rule for exercise is to start with no active quick acting insulin on board and to start with levels over 9 mmol/l, the rest is a case of trial and error and monitoring results. However my goal is to avoid the hypo as this stops exercise.
I weight train and run, both crash me, so I run in the morning when insulin resistant and less likely to go low and weight train at night after dinner with a reduced bolus and then keep glucose on me in case I crash, sadly haven't mastered weight training over an hour before crashing, but as I use a Dexcom I can stop when I see a downward arrow at 6 mmol/l.
There is just a little bit more to consider with exercise but it's about learning what works and what doesn't but never letting T1D stop you.