1. If you know the slab's manufacturer, ask them. Sometimes they will cut the hole for you for a fee.
2. Do the slabs have a grouted key along their adjacent sides? If so, this greatly simplifies things, since the key will transfer load along that joint.
3. Will the size hole you want mean cutting the prestressing strand? Will it mean cutting one slab completely across or will the hole span part of two slabs?
4. If you cut the slab completely across its width, you will need a steel "hanger" to carry the two sides of the cut slab. The manufacturer has standard details for these.
5. Don't forget to shore the slab from below BEFORE cutting to support the ends while you install the hanger.
