Navigate to the "Trustless" tab and make a pledge. Here you must decide how many DASH to stake and for how long.
Once you have made your pledge you must use the dash-core software to generate a signing address for the next steps. This is just a regular DASH address which you can make using the getnewaddress command.
In case you don't have a synced dash-core there is no need to wait for the whole blockchain to download - in fact dash-core doesn't even have to be online for what we need to do.
Once you have your signing address, you extract the associated public key using the getaddressinfo command and then use the signmessage command to confirm that you are ready.
After this, you have to wait for other participants to join to complete a 1000 DASH pool. Once other participants join in, the pool is locked (meaning that you cannot withdraw your pledge).
You will get an email once the pool is complete and you can go on to the second step.