How to make bread with plain flour?
You don't need bread flour to make decent bread, plain flour doesn't generally have as much gluten content but it has enough to make bread that's perfectly fine. You won't get the same texture as with bread flour, it won't be as elastic a result. 

Gluten is a protein, strong flour has more protein in it than plain flour, but there isn't always that much difference between the two. It depends on the brand. In my cupboard my plain flour has 10g of protein and my strong 12.5g, that's not that much more. In the past I've had plain flour with a lot less, like 6g, so your plain flour may have more gluten than you think. 

If your plain is a little weak you can use your strong flour to bulk up the gluten content, a 30/70 mix will give a small boost and a 50/50 mix more so. But, remember if plain is all you have you can still make bread. You may want to reduce your hydration a bit with lower protein, and give it a longer proving time to develop the gluten you have.