Start with The Longest WallLets start for example in a bedroom. The first place I start with will be the longest wall. Just for this sake we will do one piece at a time, when you get the hang of baseboard installation you will be taking multiple measurements before going to the miter saw.
Take out your tape and measure from corner to corner being extra careful to get an exact measurement from sheetrock to sheetrock corners.
Pick out a nice piece of baseboard trim for this measurement and at the same time pick out a couple more that have a close wood grain pattern and color. The wood grains and color can vary quite a bit even though it is the same wood. For quality baseboard installation you want to try and match this as best as possible for a good look to your woodwork.
If you have an undesirable piece of base in your pile, grab that also and head for the miter saw. At the miter saw set your undesirable piece or just any piece of baseboard trim on your miter saw with the finished part out towards you and the back against the fence. Adjust your tables or rollers or what ever your wood is resting on so you can hold it firmly against the fence of the miter saw with total control. It is important to have a good firm grip on the wood while you are cutting it so as not to injure yourself. Cut 5 or 6 pieces about 3" to 4" long and put them in your tool belt for later.
Now grab the nice piece of new wood you picked out and set this on the miter saw. Cut the very end off so you know you have a nice square cut end. Transfer your measurement from the bedroom to this piece of baseboard and add about an 1/8" to your measurement. Cut this piece and head for the room where you are installing baseboard. Take the remainder of short base pieces and spread them out 3' to 4' apart against the wall flat on the floor so your new baseboard trim will be up on top of these and off the floor providing a space for the carpet to be tucked under. Next find the center of at least one stud in the wall which your base will be nailed to and put a pencil mark on the floor marking the center of this stud.
Now assuming you measured and cut your piece of baseboard correctly it should be a shade long.
Put one corner in and bend the wood so you can get the other end in against the wall in the opposite corner. Your piece of baseboard trim will be slightly bowed out so all you do is slap the middle in and it will stay. Make sure the new wood is resting nicely on the short pieces that you laid down, if not just lightly tap it down where ever necessary and you are ready to nail.
Start your tape on this mark and put a pencil mark every 16" in both directions to the corners. This is assuming the studs are 16" on center and that is usually standard. Sometimes the studs may be 2' on center and this does happen quite often in a lower level or basement of a house which is being finished.
Grab your big finish nail gun and shoot a nail in about a 1/2" down on the baseboard at your marked stud locations.
Check to see if you are hitting the studs by slipping your little 7" wonder bar underneath the baseboard and gently prying up and out. A stud finder is a nice tool to have at this point and relatively cheap for saving time while doing the baseboard installation.