What is exactly the Cisco's E1/T1 time slot?
How do I configure them?
The time slots are inherent to the (channelised) E1/T1 transmission method. An E1 has 32 time slots. One is used as a clock and is not usable for sending payload (this is, I believe, time slot 0). When used as an ISDN30 link, the E1 also has one time slot (slot 15) dedicated as a control channel (running Q.921 and Q.931 for ISDN signaling), but when used for data transfer, this is normally available for payload.
It is configured in a way similar to this (I shall happily assume that you have a channelised E1 controller in slot 0/0 for the purposes of the example):