Before you start hacking away at your freshly bought lumber, take the time to figure out what you want your finished project to look like. An extra half hour spent on this step will save you loads of work, money, and aggravation in the end...trust me.
It helps to measure out the area you want to use. Then go up from there. Once you have your final design, you can start to measure out the total amount of lumber. For this project I kept the divisions simple at 1', so i simply counted them up, accounted for the wood thickness* where it was needed, and got about 35.5' in total. At this point you may want to break down the lumber into individual pieces you want to buy.
So, instead of thinking "ok i need 40", so i'll just buy 4 10' pieces", do a little math and actually map out what size pieces you need and then price out a few different options. After getting the measurements on EACH piece, i figured i could get all my cuts in with 6 6' pieces for the least amount of money. This will greatly help you reduce waste wood in the end and may save you time and money from going out to buy extra.
*the difference between nominal wood sizes (2x4, 1x8, etc) and the actual wood sizes will vary from piece to piece due to moisture evaporation and planing the wood flat. Typically, you can expect anywhere from 1/4"-1/2" difference between the nominal and actual sizes (see here
for a chart on nominal vs actual wood sizes). For this reason it's always important to keep that in mind when planning a project and to measure the lumber once you buy it to verify.