In order to cut evenly sized planking stock, you need a way to make the same cut on a lot of wood and get reliable and repeatable results. Cutting by hand will include too much variability, for a job like this, you will need to use machinery that is set up to mill the lumber over and over without change.
To guide the planks through the bandsaw, you will need a rip fence; which is nothing more than a fixed wall that you slide the wood past. The rip fence is set next to the blade, and the gap between the fence and the blade will be the width of the resulting board. The kerf of the blade will be deducted from the stock, so it doesn't need to be factored into the measurements at this time.
Rip fences are available from hardware stores for a significant price. Most bandsaws will sell you a model specific rip fence that will fit their unit perfectly. The alternative is to make your own for cheap!
In the simplest of forms, a rip fence is just a vertical stop that is set square and perpendicular to the bandsaw's deck. All you need to do is attach a flat and vertical board to a flat and low board. Screwing them together in a square will ensure that your rip fence is square. If there is any discrepancy, a few passes over a jointer will restore this ideal right angle that you are searching fore.
With the rip fence assembled, you simply need to attach it firmly to the deck. Simple C clamps work well for this. The compression force the exert on the fence will remove the possibility of any movement.
The concept is very simple: clamp the fence to the deck at the desired distance from the blade, then pass a test board through.
The test board needs to be measured for consistency. If the bandsaw blade is too slack, it will bow out and result in a thicker and off angled board. If the blade is too tight, it will snap! You always want your guides as low as possible and the blade as slack as allowable.
With everything setup and verified for accuracy, you're ready to start milling your lumber!
With a rip fence on the bandsaw, resawing planks is very easy. All you need to do is push the boards through the saw while keeping pressure on them against the rip fence. It is always a good idea to use other sticks or boards to push the wood you are cutting. This is especially true of the last few inches of the cut. When the blade exits the end of the board, it will continue to cut whatever it comes in contact with. If your fingers are pushing the wood through, your blood will stain the wood as your fingers are sliced open (and possibly off) by the blade.
A push board will allow you to safely push the wood through the blade without any fear of amputation. I prefer to use a large block from the scrap pile. When it gets too cut up, I just switch it for another scrap piece.
Now you can safely mill your own planking stock instead of buying loads of expensive boards from the lumber yard!