You'll need an old bra, a seam ripper, scissors and either a needle and thread or a sewing machine.
Start by picking out an old bra (this is one I grew out of years ago and for whatever reason hung on to. If you can, get a bra that has the same number of hooks and eyes as the bra you want to extend.
Using your seam ripper remove the hooks and eyes from your bra.
Next pin your hoods on the back side of your eyes and sew.
Hook your hooks to the last row of eyes on your bra and wear as normal.