Insomnia-I;m sure posters will have diffrent answers to your Q and different methods, I'm certain, work .
Once the spuds has been planted, I fill the bag half way with compost, about 9inches, bearing in mind there are stones in the bottom, then there's a layer of compost to lay the spuds on. When they start showing leaves, I cover again with about 2 - 3 inches of compost and keep doing this until the bags are full to about 5 inches from the top.
If the leaves grow to several inches it becomes more difficult to get the compost under the leaves without breaking them and NO do not firm down the compost, you are likely to break the roots/plant stems! which is what the new spuds grow from.
Alternatievly, some posters fill the bag right up to the top after planting the spuds.
Nows thew time to put in about 4 to 5 short canes into the bag and thread twine around them as a support, the foilage does get quite high and a strong wind will break it without the supports.