Putting more dish into a wheel...how to do it?

As my old 6 speed freewheel was knackered I've gone to a 7 speed, and although there is plenty clearance between the dropouts, it looks like I need to put a little more "dish" into the wheel, in effect keeping the hub where it is but moving the rim more to the driveside. I guess this is "simply" a case of lengthening the left spokes first and then taking up the slack on the drive side of the wheel, is there any specific method of doing this or a good rule of thumb for how many turns on the spoke key?

And also, before I put the freewheel on, should I be putting one of those plastic guard thingies between it and the spokes or do people not bother?

Thanks again!

Do everything in half turns, start at the valve and untighten the non drive side first by half a turn a spoke (don't start by tightening the drive), then tighten the drive by half a turn. Take wheel out and lay on floor, now gently apply pressure on the rim all the way around, flip wheel and do the same (you should hear a few pings as the spokes untwist themselves. Pop back in frame and check to see if it's enough, if not repeat. Now true the wheel - don't true just by tightening spokes, loosen opposite off a small amount and tighten other by a small amount.

Only thing I would ask is if you are putting on a 7 onto a 6 speed wheel have you put additional spacers on the drive side axle and removed some from the non drive side? If you don't then the chain will foul the chainstay when in the smallest cog.

Ignore plastic thing, just adjust the mech limiters properly

Great advice, thanks. Now I look at it, it never had enough spacers, and the rear triangle was being forced in all the time. I'll have a play with some spacers and I might just get away with only a very small tweak to the dishing, if any at all is required.

Thanks again, and good news that no plastic disc is required, they look crap I think!

If you haven't already started it's easier if it's a slight amount to just move the position of the hub relative to the axle.