I'm not talking about midi or synthesizers. I have a Digitech Rp 500 processor. I want to know what settings I must do to make my guitar sound (atleast in a rough way) like a sax.
First, of course, is to play single notes. The Sax is a single-voice instrument, and double stops will not sound sax-y.
Next, you'll have to change the attack-decay-sustain-release characteristics of the guitar to match the saxophone. The sax has sustain as long as the player has breath, and there are techniques like circular breathing that expand that. You can use an eBow to match that, but then you lose the attack. Some compression or overdrive might help here.
The opening to the Byrds' "Eight Miles High" is Roger McGuinn's attempt to transfer John Coltrane's intro to "India" to a Rickenbacker 12-string. This shows the attack that saxophones have and the difficulties in getting that on guitar, but it also segues into phrasing. Saxophone players have a way of playing that guitarists, who don't need breath control to play their instrument, find foreign.
EQing the tonal qualities should be the easiest part. Getting your guitar and fingers to behave more like a sax would be the harder challenge.