Weaving involve warp and weft, warp threads in the picture above are blue and the weft is red.
To start weaving on your cardboard loom thread the warp thread from back to the front in the first slot. Take the thread to the opposite slot and place it in the slot, wind the thread over to the next slot and bring it back to the front. Keep threading your loom going up and down until you reach the other side.
Stick down the two ends at the back of the loom with some sticky tape.