"Safe" Notes
Grab a jam track and matching harmonica to play in 2nd Position (A Harmonica in the Key of E for example). Check that the jam track starts on the I Chord (one chord... beginning of the form), if not, wait to play until the introduction is complete (visit "Performance Training: Openings" if you're not sure you're hearing it correctly).
1) When the jam starts, play any draw note for the I Chord (the first four bars of the form). Mix the draw notes in any way. When the IV Chord (four chord) comes on bar 5, play any combination of blow notes. It matches. Maybe what you played didn't sound inspiring, but it DID match.
2) Play a lick that starts on a draw note... hold that draw note and then improvise a bit. When the IV Chord comes, play a blow note and improvise for the rest of the two bars. When the I Chord comes back (bar 7), start with a draw note again and then improvise a bit. There's four bars left... If you want to complete the form, stick in a cool V-IV-I Lick you learned from Improvising Study #4 and then a cool turnaround lick from Improvising Study #5.
Our second example is what experienced players commonly do... tip their hats to the chord change. You don't have to... nor do you want to... limit yourself to only the notes of the IV Chord, but it's a good idea to hit one (any blow in this case), so start with a note that really sounds good with that chord and then let your improvising skills take over.