I'm assuming it's full-length outer cable you're having trouble with.
Start with an inner cable. Place the cable along the outside of the frame with the end near one feeder hole, & grab the cable near the other hole. There you have a reference of how much cable you need to feed into the frame to find the exit hole. Make a slight kink a couple of centimetres in from the end & feed that into the frame. When you get to your length mark (where you're holding it), twist the cable around, sliding it back & forward a little bit so you can feel around for the exit hole. When the cxable pops out, pull a bit through so it's not going to retract itself. You now have a "mouse" or leader to guide the outer cable through the frame. When you've fed the outer, remove the mouse, cut the outer to length then feed your inner cable through properly.
If it's a segmented outer cable, feed the inner & outer sections from the lever as they need to be, then it's the same process to feed the inner through the frame. Where you'll find trouble is that the exit hole will have a step inside the frame where the outer cable stop is. The blunt fact is it will be an absolute pig of a job to get over that step & find the hole. There are no tricks aside from that kink in the end of the cable, which will encourage it to feel the wall of the frame; it's simply a matter of persistence.
Just ride the bloody thing and enjoy yourself!