Sounds so simple doesn't it, but like all building work it takes practice and a little know-how to get a straight line.
Look at the teeth on a saw. They are "set" at an angle to each other. This allows the saw to cut easily through the wood leaving a cut wide enough for the saw to get through without sticking. This only works if the saw is going in a straight line. If the cut you are making veers from the straight in any way, by even a few mm, the saw will stick.
There are two very basic faults which turn a relatively simple job into a nightmare. People do not secure the timber they are cutting and do not get right over the top of the cut.
Firstly, it is so important that the work, and work bench is solid and secure. One little rock and the saw is off line. Make sure you have created a solid platform to work on. If it is possible to clamp the project, then even better, but make sure you get some weight on it to stop it moving. Click on the image below to buy the bench at £19.99
Secondly, get right over your work. Ideally you should have an eye either side of the saw blade. This should ensure that your cut is at 90 degrees to the timber. If you are over to one side of the work, your saw will be at an angle.
Stroke the saw gently, let it do the work. You are only there as a guide. New saws cost very little these days so make sure you have a new, sharp one for your project. Its not worth spoiling the ship for a ha'peth of tar. Old, blunt saws have to be forced through the wood and that just leads to trouble. Click on any of the images below to buy.
Using power tools to make your cut certainly makes life easier, but until you are used to them, unlike a hand saw, one tiny slip can ruin a job. A slip with a hand saw can cause a little cut in a piece of work which, most of the time, can be sanded out. A slip with a circular saw is a huge cut half way across a door!