You did not clearly state what trains you intend to use, but Zag->Bp has just one. the DAYtime trains in all 4 are only with optional RES-except of course €* in the chunnel to LON. Now after the main hi-summer season is over, there is not much real need to RES for first 2. YOu will find a seat.

Od Prahy do Colonia/Köln is always with change-as such on any route its optional, but on DB/German ICE (I used one this afternoon)its very easy to do via bahn.com/seat only. Also for ICE K->BRU-but DO be aware that a RES will often turn out to be worthless as very often trains are cancelled very last minute (its special stock for 2 currents)-this will also mean missing that €* for direct transit.

The only thing you have to take care of now is that €*-and do that quickly.

Canucky with your maple leaf, I´ve never heard the word ´format´ for train trips, so its not really clear what you mean with that. In general a RES simply comes as a mail stating day/train/car/seat nr. For the €* it will also have a barcode to pass gates/checks-you can store that on fone or print. IF you´re out for some local experience and pay even less-you could even visit the stations in Zag and Bp (right after arr.) find right counter and let them arrange it for you-you pay the odd € or so and get a nice paper in local language but will also clearly state what would be in the mail. FREE souvenir!

You don't need to make the reservation in any particular order and the advice is to avoid making the reservations through the Eurail website if possible. Please read more below.

Here is some useful information from the experienced travellers in the Community regarding both planning, reservations and activation of pass and travel days. 

 Planning

The rail planner is normally not up to date, as it only is updated once a month, so to be sure of the time table you better check the timetable and availability on the websites of the national railways. The bigger national railways, like DB (Germany) SBB (Switzerland) and ÖBB (Austria) cover several countries. 

 Reservations 

The advice from the experienced travellers in the community is to use other ways to make reservations than the Interrail/Eurail website.  You can look at the guide in the link:

https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105

If you, after having looked at the guide, have questions about how to make specific reservation, please give your travel details (departure date, time and route) preferably in a new topic, and you will get advice.

Please note that Interrail/Eurail charges an extra fee of 2 EUR per person and train in addition to the fee for the seat reservation.

 Activation of pass

During the activation process, when you choose the start day of the validity of the pass, the first day of the validity period is automatically made a travel day, even if you don't enter a journey, the advice is therefore not to activate the pass before the first travel day as you only can deactivate the pass before 00.00 on the day the validity starts. If your travel plans change in the last moment you will loose travel days if you have activated the pass in advance.

It can be wise to make a test and activate the pass with a start date well in the future and then deactivate the pass immediately, just to see that everything works.

 Activation of travel day

The advice from the experienced travellers in the community is also never to activate a travel day, that is connect a journey to your pass and create the QR code, until just before boarding the train, otherwise you might loose a travel day if your travel plans change in a late stage  You can't delete a travel day in the past. A travel day can only be deleted until 23.59 CET the day before the travel day.

@jadelson I assume that you plan to buy a Eurail pass as you ask your question in the Eurail community. 

Yes, I do plan on using Eurrail

I strongly recommend a reservation from Cologne to Brussels - It is a very popular train and starts in Frankfurt.

I travelled it last week, By Cologne it was manic and nobody without a reservation would get a seat - stand or sit on floor only options.

Although it was on time leaving it had to be diverted (problem on line) and was over an hour late into Brussels.

WE stayed overnight in Brussels before E* next morning so no problem for us.

Hello! It depends as some of them can be locally made at the train station. If you are planning to travel with the Eurostar though, please make this reservation well in advance as availability is...scarce! :)