If you would to get a block from your wide variety of blocks in your inventory and put it in your 1st slot, then(using numbers) hover over the block and hit "2" then hover over it again and hit "1". Now exit your inventory and place the block.You will notice that the block disappears from your inventory and tries to be placed.

Sorry, but you'll have to do it yourself cause video is too big and with pictures, it's disbelieving.