remove the bottom bracket cable guides. remove any hardware bolted to the frame and the rear plastic “banana”
use an inner brake cable (its much stiffer than gear cable) and start at the top of the downtube. Feed the cable through until it pops out of the BB area. then slide a long length of internal cable routing “pipe” onto the end of the brake cable (it will be a snug fit) and drag the cable back up the DT taking the pipe with it. Tape the end of the pipe where it comes out of the DT, so it does not fall down once your remove the cable.
you can now install the hardware and feed your inner gear cable through. Always suggest doing the rear mech first. for the chainstay its a little tricker. Rotate the frame / bike in the workstand so the back wheel is up in the air (vertical). Take your inner brake cable again and start next to the BB, feed it up the inside of the stay. Sometimes you have to twizzle the cable as you feed it, other times you get lucky and it finds the exit hole above the dropout. Then plug in more long pipe and drag backwards. Tape the end next to the BB, and remove the brake inner cable
Then feed your gear inner from the DT / BB area into this pipe after threading the wire through the right cable guide (some Roubaix use a single cable guide under the BB). Once the wire is through and out the dropout, you can pull the pipe back out of the dropout and trim it to length, as it will be too long. Put the pipe back onto the cable, install the banana and finally the loop of rear gear outer
Sometimes you may have more success in feeding the inner brake cable the other way, but you tend to have to chop off the brake nipple, whatever works is worth sacrifcying a cable
Front mech is easier as you just have the procedure for the DT. Once cable is through, run it through the left cable guide and then bolt both guides in place using the cover plate and bolt
If you ever find your frame is clogged up with muck / grassy stuff / etc. I’d strip the frame, flush with warm soapy water, rinse with warm clean water and dry overnight before rebuilding
I work in a Specialized Concept Store as a bike mechanic, and have worked on 300+ warranty / crash replacement / service jobs on Tarmac, Venge, Roubaix in case you are wondering 😉