To give me a better idea of bikepacking.....what and where do you put you gear on your bike?
I reckon you might get as many answers as there are users
But for what it's worth these are my thoughts:
You've 3 main places to fit stuff, the bars and below the saddle are the easiest and the places where you can usually carry most kit, a harness of some kind will make life easier and the load much more stable but it can be done without. The third spot is the frame triangle but this will involve the purchase of a dedicated bag ... I'll include the toptube within the frame too.
A drybag strapped to the bars should be capable of holding much of your lighter bulky gear, eg sleeping bag, spare clothes, sleeping mat, etc. 13l - 20l seems to be the normal size for most.
Add a second drybag under your saddle for shelter, bivvy bag, stove, food, etc ... don't go mad, maybe 8l - 12l.
There's also the option of fork mounted cages for light(ish) stuff ... works for some, not for all.
If your frame has rack mounts a cheap/practical option is a rack but don't use panniers. Just strap your drybags to the rack.
I have all sorts of dry bags etc and I intend on experimenting with them to see what works best....
This is possibly the most important thing to do.