The best way to get back at an ex whether it's a girlfriend or your ex-boyfriend is to take it slow first.
Some people advise is that the first thing you do is you right hand written letter expressing that you accept the way things are right now.
It's important that you communicate this.
It has to be sincere and genuine or else you could ruin everything.
After you send the signal that you accept the current status, you want to give it a couple weeks and then try to see where things are.
Here's what I mean.
Set up a casual coffee meeting and try to have fun.
If it's not coffee it could be lunch but make sure that you communicate this is a casual meeting and are not trying to make it a date or anything like that.
After a couple casual coffee or lunch or pizza means you might want to try something a little bit more adventurous like go hiking or rollerblading, ice-skating or something else that might be fun.
Try see if there's still interest there and if there is you will see signs of it.
If you try to take things too fast you might blow your chances but on the other hand you have to watch the body language and communication to make sure there is interest and when you're pretty sure there is interest and you can start to go from there and maybe ask for a date.
But pay close attention and try to look for the right signs.