You need to have a War Bloc built first.

Then go to the location of the intended target and you should see "Onslaught" as an option.

Once you select that, you will be able to add the troops that you, personally, wish to send (limited by the number of troops you can send per march. Yes, 100% March Count item affects this)

After you have selected the units you wish to add and confirmed, the Onslaught will be visible to any clansmen whom also have a War Bloc. Their marches will also be limited by their normal march capacity (adjusted by items). 

However, the TOTAL number of troops in the Onslaught can not exceed the amount of units indicated in the War Bloc of the person who sets up the onslaught

Example:

Member_1 has L20 War Bloc

Member_2 has L10 War Bloc

The total troops allowed in the attack will be higher if Member_1 forms it than if Member_2 does. However, regardless of which player sets up the Onslaught, each participant is still limited by their normal March Count.

Note: Your Peace Treaty will drop when you initially set up the Onslaught. It will not, however, drop when the troops are all gathered and begin marching on the target. (So if you set Onslaught to start 8 hours from time it is formed, your shield will drop when you start it, but not when the fathered troops march 8 hours later)

Note:  Any troops that are committed to the Onslaught but do not reach the Town in which the Onslaught was formed, will not march with the rest (If you set up an Onslaught to march in 3 minutes, and 4 members add a bunch of troops but they are 2 hours from you... your troops will march alone and likely get destroyed)