Breaking a farm horse?

My best friend lives on a farm with 5 horses. There is a shetland, a really nervous pony, and 3 big tame working horses (2 males, 1 female) between the ages of 6-10. We want to break her fixed male, named Curly. He is really trusting and calm. Her uncle has hopped on him bareback twice and rode him without a problem but that is it as far as riding goes. He is just used to pull farm equipment. My friend and I are both 16 and want to do are best at doing this properly and are willing to take as much time as needed. I was just wondering if anyone had any pointers?
Answers:
Big farm working horses are not ideal for riding . They are too broad for comfort and their trotting action too big and bouncey . If he is quiet , can take the Bit , and has been mounted bareback ; find a suitably wide saddle " That fits him " . This is very important , because if it doesn't , he will be uncomfortable , get sore and may kick and buck . it is important not to upset him in any way , but gently and kindly insist that he does what You want . When you saddle him , you must already have on the bridle correctly adjusted and a long rein attached , through one D ring onto the other . When the saddle is on and the girth adjusted , I suggest a girth with an elasticated end ; have somebody hold his head , while another mounts him . Ideally you should be in a closed sand ring , but a small field will be ok . If the horse remains quiet , the person on the ground should lunge him at a walk , in a circle first one way and then another. If he remains quiet and obedient , on another day he could be long reined with a rider on top . Daily lunging with the saddle on and stirrups secured is always a recommended routine , be sure to keep behind his eye , more level with his hindquarters . have him change direction after 4/5 circuits , coming into the middle to be petted by you and told good boy . When you have gone though these routines for several days , you might consider taking him out on the road ; if you can in the company of a more experienced and well behaved horse he can follow . If you can find it an excellent guide as to how to do it is " Riding For The Rest Of Us " by American author Jessica Jahiel . ( Waterstones UK )
You say you are willing to take as much time as is needed . Quite right , don't rush it , but do something every day , if just 20 minutes of lunging , no more . I get the impression that this horse is effectively broken to the shafts as a draft horse , so the initial breaking in is done ; otherwise you should be thinking of 6/8 weeks . I hope this helps ; Good Luck .