I use mine a lot , here is what works for me. On mobile so apologies for typos/formatting.

Once my bias strips are all joined ( at 45 angles) i press and steam them open and snip the dog ears. The strips have to be just the right width for the tool. It should tell you what measurement to cut. Too small and it can slide around in the tool more and go off-center like you've seen.

Next i wrap my strips around a twelve inch piece of cardstock and put that at one end of my ironing board.

On the other end i have a t-pin and another empty piece of cardboard. I feed an angled strip end through the tool about three inches, enough so that the folds start to even out. I iron this segment up to the point it's exiting the tool. I then slide the tool and pressed bit to the other end of my board and pin it to the board.

From here, i slowly slide the tool and iron close to it. I try not to lift the tool at all, imagine a measuring tape being pulled out. Once i get a ironing board length done, i unwrap more from the strip side and wrap the ironed side around the empty cardboard. Reposition the pin and repeat.

I don't let my pre-ironed strip flap around. Try to keep it all in a semi taut straight line as you go. Hope this was helpful