Read my post carefully. i am advocating for a Flex Pass, not a Swiss Pass which specifically and only refers to a consecutive-day pass. The flex Pass is a type of Swiss Pass but the different name is important.
OK, now, to use it. With a consecutive-day Swiss Pass you get it validated and the start day stamped by a Swiss train agent before you board your first train. You can do this at the Zurich airport. Same is true with a Flex Pass--they will validate it and stamp or indicate the start date. ( The 3-day Flex Pass is good for any three days of your choice within a 30-day period).
With a Swiss Pass,once validated, you just board the train and show it to the conductor when he or she comes around. With a Flex Pass, it will have three little boxes where you enter each date as you use it for fully covered travel. Write the date in the box in European numbers (I.e., September 7 is 7/9/14, not 9/7/14). Write it before you take your first train of the day or they can accuse you of trying to slip in an extra free day. Then just show the pass to the conductor. BTW, with a Saver Pass (15% discount off the pass price) you and your travel companion will be on the same pass and must travel together when using it.
If using the Flex Passmon a "non-covered" day to get the 50% discount, whether on a train, boat, or cablecars, just show it and ask for the discount when you buy your tickets. Do NOT write the date in one of the boxes to use it this way.
To use either pass for mountain lifts that offer the 50% discount, just show it when you buy the tickets. Most of these lifts have gates or turnstiles that you cannot pass without a ticket.
You can make reservations for seats when using a pass but it costs extra, about 4 CHF when I last did it. You do not need them at all for Interlaken to Spiez, no point. They may be mandatory for the train that crosses the border, whether that is Spiez-Milan or Brig-Milan. If not required, I wouldn't make them, but that's just me. Your Milan to Monterosso ticket for the IC train will include a seat reservation at no extra cost.