Start now. Provide a litter box big enough for the bun to fit in completely. Use litter only in the litter box. Litter placed in the cage confuses them into thinking the whole cage is a litter box. Either hang a hay rack above the litter box so she must be in the litter box to munch on it or place a pile of hay in one corner of the litter box as buns potty while munching.
If there is a certain area of the cage where she seems to prefer to potty then place the litter box here. Any poo outside of the litter box, pick up and place it in the litter box. Any pee outside the litter box, soak a corner of paper towel in the urine and place this in the litter box. Clean any surfaces that have been peed or pooped on with a mix of white vinegar/water. The vinegar is safe for buns and completely breaks down the smell of urine so your bun will not return to a specific spot she has pottied at before based on smell. If you do not like the smell of vinegar then once you have cleaned with the vinegar/water you can then wipe everything again with a damp rag using just water.
Restrict your buns space until litter trained. Buns are considered litter trained when they will pee only in the litter box. It is very rare to have a bun not drop at least some poos outside the litter box. Once litter trained then you can slowly allow your bun more space. For a larger space you may need to provide an additional litter box outside of the cage.
This has worked for training my three buns, my male at ten weeks old and the two female buns I rescued from a neighbor at 1 1/2 years.