Hallo there Jlasud!
I congratulate on your efforts, and hope that you continue in that way: making a truly working tool, or weapon, using ancient methods is a very, very, very big satisfaction!! You seem able in manual works; continue that way and don't be afraid of failure, because bad meltings can always be melted again, and again, and so on
But I think you will achieve it the first time!
About the bellow, it's very easy to make one. I'll try to describe how I made mine.
Basically, it's a square, with each side long 60 centimeters (23,6 inches I think), made with soft, but resistant, leather.
So, I took a piece of leather long 1,20 meter and 60 centimeters wide (47,2 x 23,6 inches) and folded it; I then have sewed its sides. But be careful: on one side, near to the bottom of the bellows, leave about 5 centimeters (about 2 inches) of leather, without sewing it: there's where you will insert the pipe.
Once you've sewed all, apply four wooden sticks to the top of the bellows: viewed from above, they should form a shape like this < > : they will be the handles with which you will open the bellow, put the air in, close it and then press the air out of the pipe. You can apply those sticks by folding the leather around them, and then sewing it; I've practiced holes at one edge of each stick so that I've been able to tie each couple; this prevents them from moving.
Then, choose your pipe: for the bronze, we've used ceramic ones, but it's an error, because no such archaeological foundings are documented. When I've had to make my bellows, I wanted it fully philological, so I took a cane, found with dozens of others along a river, holed it inside and applied it to the bellows, inserting it in the "hole" I told you about above, and assuring it with ropes.
Now all you need is a nozzle, which I have made of clay. It's very resistant, it just melted a bit when I melted iron (but it mean that it can bear a 1500° heat, so no problem with bronze).
Note that the wooden cane tends to burn slowly (I change one each 20 or 25 forging), but it's easy and cheap to replace it.
Now you have a historical bellows, which requires less than 10 hours for making; I even know how to built Celtic and Medieval ones, but ask me via PM if you need
If my explanation is not clear, ask away and I'll try to post some pictures or paintings about making a bellows!
Greetings and good work,
Mauro.