How do you choose side missions?

Look up "Building the Side Mission Deck" on pg 17 of the RRG

Shuffle all red side missions and 4 chosen green side missions along with 4 random gray side missions.

After aftermath, shuffle the deck and place two in play. Rebels will select one mission to play. Remove that mission and replace it with new card. The next time Rebels select a side mission, they choose from the two in play.

Ok, thanks

Look up "Building the Side Mission Deck" on pg 17 of the RRG

Shuffle all red side missions and 4 chosen green side missions along with 4 random gray side missions.

After aftermath, shuffle the deck and place two in play. Rebels will select one mission to play. Remove that mission and replace it with new card. The next time Rebels select a side mission, they choose from the two in play.

Also worth noting the bit in the Learn to Play guide that the ONLY red cards you'll have in your side mission deck are the ones that correspond to the heroes the Rebels are using. In a non-custom campaign, you'll never have less than two and never have more than four red side mission cards in the side mission deck.

IIRC the core game comes with a total of 14 side mission cards: 6 red (one for each hero), 4 green and 4 grey. So the rebels don't really get to choose which green missions go into their side mission deck unless the OP has purchased expansions or packs that have extra side mission cards.

Lastly, probably also worth pointing out that page 17 of the RRG documents how forced missions are played. It's a slightly related topic, but I'd guess the OP might have a question about that if he or she is asking about side missions generally.

Bearing in mind that its only the red missions that correspond to the chosen heroes (max 4), 4 random grey (you don't look at them), and 4 green missions (that are chosen by the rebels).

In Return to Hoth - its only 2 grey and 2 green, as the other 2+2 = 4 cards are replaced by the threat missions.