I have specified the trains to catch in previous similar questions.
To save you using the Search feature which is a bit complicated on this site, here's the text:-
Use London Midland trains which are scheduled to stop at Watford Junction
((this only applies to London Midland trains, Virgin trains at similar times and/or to similar destinations or stops do not serve local passengers between London and Watford Junction))
Check the screens at Euston to find the trains I talk about below. References to the platform numbers is giving the platform that they arrive on at Watford junction.
most Tring trains go at about xx:34 or xx:05 and usually call at (sometimes Wembley Central), Harrow and Wealdstone, Bushey, and platform 8 at Watford Junction.
most Milton Keynes trains at xx:24 are first stop Watford Junction platform 8.
most Birmingham trains at about xx:49 are first stop Watford Junction on platform 6 (the fast line).
most Northampton trains at xx:54 are first stop Watford Junction on platform 8.
At rush hour 4:30 to 7:00 pm there are some extra trains. The line is extremely busy and standing room (very tight standing room) to Watford Junction at rush hour and I strongly advise against traveling at that time.
Don't board the xx:13 to Birmingham or Northampton or xx:46 to Crewe because they don't stop at Watford Junction.
When you get to Watford Junction go outside the station and cross the forecourt to bus stop "D" where the well labeled stop is where the Knight Bus stops. You can't miss it.