To make use of conditional blocks, you will need :

1 - one value prompt

This prompt will allow the user to choose the conditional block to display. It can be located on a promt page, on the reports pages themselves or both.
On this particulat prompt, set a parameter name, auto-submit to 'Yes' and define a static list of choices.

2 - one variable for conditional choices

in menu view->variables, create a string variable to be used as conditional variable in the conditional block. and set Expression to get the prompt choice parameter value.
Create as many values as conditional blocks you need, keep the same ones as in the conditional prompts.

3 - one conditional block

On report's page, add a conditional block and set the conditional variable to the one you have created & select values you vant to use.

Then, you only have to select one value in current block, add contents to it. Select another value, add contents... it's done.