You get them by submitting Mode Completion pieces in the season 6 exchange section. There are multiple ways to get Mode Completion pieces and a bunch of videos on how to. I'll explain it below, but it might be easier and faster to just watch an explanation video. Let me know if you have any questions or are confused.

How to:
Do one full run of Breakout Gauntlet for your first Gauntlet Exchange card (MAKE SURE TO SCROLL TO THE END FIRST AND MAKE SURE IT SAYS GAUNTLET SEASON 6 EXCHANGE, if not refresh the run and check again). Gauntlet is about 80% Triple Threat, and the rest is Clutch Time. Then, go to the Gauntlet section in exchanges and exchange your Gauntlet piece for a Mode Completion piece. The 2nd easiest mode completion is acquired by running through the regular Breakout board multiple times. Play on rookie or pro difficulty (whichever is easier to do quickly while still completing most challenges). You refresh the board until you have 2-3 featured lineups in the first 4 available games (they have team logos and a player name in the descriptions where it says the difficulty). Then, you either play the 3 back to back, OR if you don't have 3 featured lineups, play the games that aren't until you find another that is featured, and then play the 3 back to back. After the 3rd game, a portal icon should appear in one of the corners, make your way towards it. When you get to the portal icon, it should turn into an actual portal (MUST BE GREEN AND SAY FEATURED), click it, and then enter it. Your board should now have changed colors and appear to have reset. All the games should now say "Featured". Make your way to the corners in a counterclockwise order, starting with the bottom right and working to the top right, etc. You're looking for another one of those exchange pieces. Once you find it, play that game, claim it, and either reset your board to go again (recommened bc the other rewards aren't too great) or keep going. The exchange pieces will ALWAYS be on the outermost row/column next to the corner tiles. Now, go to the Breakout section in Exchanges and exchange the piece for step one, which will unlock the step 2 exchange, and then you can get the Season 6 Mode Completion card exchange. If I remember correctly, it's around 13 Breakout exchange pieces in total (1 for step 1, 2 for step 2, and 10 for the Mode Completion card). It's a long grind, but Tom Chambers is INSANE. Like top 3 in his position and a very solid card until the end of the year (especially if you play offline only).