So you have a good basic understanding.. (actually my whole dilemma started because I want to add floor lighting to my foot wells because currently I have NO lighting at all that works with the doors).
And LED's usually don't need a relay/cause hardly any load on the circuit. So in your case you could just find the wire that goes to your dome light either in the left or right kick panel (in front of the front door) or easier but more work... tap a wire off of the positive wire behind your dome light, run it down along your A-pillar (beside the windshield), fish it down beside or through the dash somewhere and hook your lighting to that, just run across under the dash from one light to the next in a string. For the ground or negative wires you just attach those to the metal bracing behind the dash or any ground wire you can find under there... orrrr run two wires from the light through the headliner and down along the windshield