1) How exactly do you read a plan? I got a plan of those old simple 4 player Konami cabs (TMNT, Sunset Riders, Simpsons etc.) from Jakobud. I understand the measurements and all, but between all the lines and numbers, it gets a little confusing. Is there a certain standard to reading a plan that a beginner like me should know of?
Can you attach or link to the drawing/plan you are looking at? It would be easier to explain if I/we know the drawing in question.
2) How do you make sure every side is perfectly straight? Is there a tool or method for this? I'd hate to take out a ruler, draw out my measurements, think I'm getting everything straight and end up with a discombobulated mess instead.
It helps if you have some material with square corners (meaning 90°).
Say for example you need to draw a 400mm long line 500mm in from the side of the material. I would mark 500mm at the top and bottom of the materail (measuring from the same edge) and then draw a line to connect the two marks. You should then have a line that's parallel to the edge of the material. You can then mark/draw your 400mm length.
3) How do you accurately measure, and draw out angled sides before cutting (such as the top rear area that usually that usually slants upwards, meeting the marquee)?
Rather than try to draw the line 'as an angle', I would mark the coordinates of the two end points of the line and then join them up
4) How do you cut out rounded edges? The side panels of my planned cab has a few edges that are slightly roundish. How would I go about cutting out these round edges accurately? The plans point to these edges with with tiny notes beginning with an 'R' followed by some type of measurement, like R0.625". What does it mean? Or is there a specific tool or method to getting these edges accurately?
The R stands for radius. If you drew a full circle, the radius would be half of the diameter.
For example:

In this case the diameter of the circle of the circle is 10cm and the radius is 5cm.
In your example R0.625" would be a radius of 0.625" (5/8")
You have a few choices in how to draw/cut the radius. You could just use a compass to draw the line/circle:

The distance between the two point of the compass is the radius. You could then just jigsaw etc the bulk of the material and then sand to the line.
I personally use a router for all of my finishing cuts. I would mark the radius, cut the excess of with a jigsaw and then use a piece of material with the same radius as the arc I want to cut as a template in conjunction with a flush or pattern trim bit. To create the circle template I would use a hole saw (adjustable or fixed) or router circle cutting jig depending on the size of the circle/radius. I often make metal cylindrical templates on a lathe to cut very accurate circles/radii.
5) How would you correctly angle and measure out the cabinet's monitor shelf? These details aren't directly noted on my plans, and the monitor area for this cab is very slanted.
It will depend on your monitor and cabinet/plans. I/we could help you out once we have some more information.
Any other suggestions or help would be very appreciated. As you can see most of my concerns are with knowing how to get certain measurements and angles correct so I don't unknowingly end up cutting out a mess like a novice. I'm no craftsman, but I have a great record of teaching myself new things. I know most of these are dumb questions, but it's very important to me that I fully understand every aspect of this so I don't end up screwing up, doing this over and over again, and wasting money. So please take the time to help a poor noobie out!
Don't worry, things that you don't know a lot about can seem daunting at first but there are many people who can help you out. You obviously have the willingness to learn and want to do the job properly which is half of the battle.
One of the best pieces of advice I can give you is think thrice, measure twice and cut once which basically means think about what you are doing/are going to do, measure your dimensions and the check them. If you have done that you should be able to cut your material and end up with the right size pieces. Also planning is key, try and work out exactly what you need to do before you get to do it. If your anything like me I'm always thinking about how I'm going to do something and ideas develop as I think. Its a good idea to plan things and make drawings/notes/lists etc. This forces you to think about what you are doing and can often highlight problems you may not otherwise think about.
If you want any more help don't hesitate to ask. If you don't understand something or it isn't clear I don't mind making some drawings etc if it helps to explain things better.
Most of all, have fun
