Do enough work with hand tools and you'll come to appreciate the main handtool - a bench with several clamping/vise options. Something modern like a workmate is a start, but a bit light for the job.
The Trick (well, other than actually using a ripsaw [preferably a SHARP ripsaw] as Jack mentioned) you MAY not have thought of which makes this cut MUCH easier is to clamp the board vertically.
The saw is now "straight up and down" and the angle with respect to the board can be guided by both marking lines on the face of the board (and checking both sides) but also by placing objects or making pencil or chalk lines on the workbench it's clamped to that you can sight the saw in line with. Start with not too much board sticking up, so it does not vibrate too badly. As you cut more, pause and move the board up in the clamp/vise. When you get to the end you can either cut from the other end to meet the first cut, or do the last bit clamped differently, with the existing cut helping to keep you on track for that last little bit.
This sort of thing is such a handy trick that some workshops had little trapdoors under the vise position to permit dropping longer boards through the floor while clamped this way.
If you want a more accurate result, mark the initial sawcut away from the final position (note - handy hand tool here is the marking gauge - you can make one yourself) and then plane to different marks for the final line. Check both sides of the board as you approach the marks.
The main "trick" with hand planes is that they do not work at all if dull. They will force you to learn to sharpen them well - after which they are a joy to use.