When you build a depot (b > D) an architect has to turn up an design it before it can be built. So, make sure you have one dwarf with that labour enabled.
It's best to build a depot before your caravan arrives, because dwarves are excellent at finding other things to do when you need them to get busy with trade.
You don't absolutely need a broker, if memory serves, but you should assign one anyway. In order to gauge the value of items, a broker (assigned with the noble screen, hit n to open that) needs to have the appraise skill. If no-one has it, don't worry, it's easy to raise.
When a trade caravan turns up (which for dwarves will be announced most prominently by the arrival of the outpost liason), select your built trade depot and hit g to select goods you want to sell. Dwarves will bring the items to the depot. Dwarves will do this even with no hauling labours enabled, but sometimes dwarves seem to prioritise other tasks over taking things to the depot, so if they're not bringing things fast enough, deactivate their labours.
When you want to trade, select the depot and hit r to request a trader. By default, this will be the assigned broker. Trading is the last thing any dwarf ever seems to want to do, and in my experience brokers will insist on getting a full night's sleep, eating, getting something to drink and then going for a smoko before they actually bother turning up, even when they have no labours enabled. But, if you want your broker to trade, then you should deactivate his labours. (He may turn up anyway even with the labours enabled, but don't hold your breath).
I prefer to have any dwarf turn up to trade. The broker with the appraise skill doesn't need to be in the depot for you to see the prices of goods. Having him turn up just means he will improve the appraise skill. Hit b to select any dwarf to trade, if you wish. This makes it more likely someone will turn up to trade sooner rather than later, even if you don't fiddle about with labours. If you don't have anyone with the appraise skill, it doesn't matter who turns up; you can make them gain the appraise skill and then make them your broker. Brokers don't have to do anything special, they just let you see item values, so it doesn't matter who the broker is as long as they have at least a little of the appraise skill. But, you may get more accurate value readings if they have a higher skill.
When someone turns up at the depot to trade, if they don't have the appraise skill, then press t to trade, select an item on your list of items (preferably a less valuable one, but it has to be one you've made, which shows up in brown) and hit o to offer it to the traders for free. This makes them like you more, but more importantly, it counts as a trade and will improve your trader's appraise skill. Exit the window (so that the skill increase is tallied), and hit t to open it up again. If you offer up 3 or 4 items, (or if you wish, trade 3 or 4 items, it makes little difference) then your trader will gain the appraise skill, and you can make sensible, equitable trades. Be sure to exit the trade depot query and set that trader to be your broker. Then, you will be able to see item values, and can enter the trade screen, and trade as normal.
Trading is fairly self-explanatory, the screen isn't too hard to understand.