If your soldier in a mission is armed with satchel charges, you place the charges by bringing up the ACTION MENU and selecting PUT SATCHEL CHARGE.
Once you've placed a charge, there are two ways to detonate it. To set off a charge immediately, simply select TOUCH OFF # BOMB(S) from the ACTION MENU, where "#" is the total number of undetonated charges you've placed. Note that you can only set off satchels as long as they are within about 300 meters from your position.
The other way to set off a charge is to use a time delay. While near the charge, select SET TIMER ON (30 SECONDS) from the ACTION MENU. If you display the ACTION MENU again, you'll see the timer counting down. To add additional delay time, select SET TIMER +30 SEC. (# REMAINING), where "#" is the number of seconds remaining before the satchel charge detonates.
Even if you've set your charges on delayed timers, you can still always select the TOUCH OFF # BOMB(S) action.
To deactivate a satchel charge that you've placed, simply go over to it and select DEACTIVATE BOMB from the ACTION MENU.