Unless you have a large boat with massive rigging
, I would recommend going with ratbars instead of ratlines. Ratlines put a lot of sideways forces on your shrouds which greatly increases the tension in them which can be problematic on a smaller vessel (believe it or not but you can put more tension in your shrouds if you step on a tight ratline than your rig will on a smaller boat). Ratbars do not pull the shrouds together so this is not an issue. I find ratbars much easier to climb as you know where they will be and you can't push them out of the way accidentally(I used to work on traditional boats where you regularly went aloft this way).
To attach them, I like to use seine twine which is an extremely strong tarred nylon. I do a seizing tightened with frapping turns and have no problem keeping them in place. You can use marlin and be traditional but be careful as it rots over time. If your boat is really traditional, then tar everything, if not, I would stay away from it and use a wood for the ratbars that can be left to weather