Here is the example he experienced departing KILG for Leesburg, KJYO. I don't recall his exact clearance but this is how knowing what to chug and plug can make life easier.
On departure he was given vectors to join V469. Ok the GLOMO intersection is out so he has to add in JOANE (top arrow) second pic, activate that leg between KILG and JOANE and now the GPS and CDI provide guidance to intercept.
The next instruction was to join V408. If you look at the first chart example there is no intersection where the two airways cross, however, GLOMO and ROBRT are already entered. Following the example from our flight today we would select ROBRT (the destination for the GLOMO ROBRT leg) on the second chart and activate it once instructed by ATC to intercept V408. Your instrument flying remains precise and the amended route is not a problem.