I'd be careful about using DE in powder form in a house see
http://www.shagbarkbantams.com/de.htm. A bit on the extreme side innmy view but worth considering.
I use it to deter Red Mite in the warmer months but I aways mix it into a slurry by starting with a dilute disinfectant base with a dash of washing up liquid added to help wetting. I add the DE stirring as I go so you end up with a thin batter type consistency. I paint this on the perches making sure it gets into the cracks and into all the nooks & crannies in the house. Once dry it works just as well as the dust but isn't so dusty that it floats in the air. Dust in a chicken house is to be avoided as it can irritate a chx vulnerable respiratory system as well as transport viruses very effectively. Use as described it's much safer. Add it to a dust bath outside where there's plenty of ventillation though personally I don't see much need to do this, earth works just as well.
Don't use it as a wormer, it's of doubtful effectiveness and as others have said flubenvet is so effective, safe and not too expensive that there's no point in using other so called natural wormers for which there's little or no evidence they really work and I include some of the organic ones in this such as VermX.
All I'd add to the advice already given is that when working out the dosage remember you medicate the feed according to the weight of feed eaten. This method only works when feeding low energy layers feed. If you feed higher energy feed like treats houshold scraps they eat much less bulk as they eat proportionately to their needs. So if you used the same method with high energy feed for example you would under dose them as they eat less bulk. Hence the advice on NOT feeding treats which aren't medicated.
If your hens free range this will dilute the dose too so you can either do as Joy suggests or just add it to the layers pellets at a bit higher concentration. Typically a layer will eat around 120mg of food a day or 850mg per week so at a normal concentration of 3mg of 1% Flubenvet (the one sold for hobbyists) per kg of feed your chx should be getting 2.5mg or there abouts over the 7 days you must administer it.
If you know how much layers feed your chx typically eat per week you can make up a weeks lot of medicated feed by adding a 6mg scoop (provided) to it per two birds (a small overdose won't hurt, it has been tested at at least twice concentration without harm so it's pretty safe).
If you use pellets, in order to prevent the powder settling out, add some corn oil to the pellets and mix thoroughly so the pellets are very slightly darkened (you won't need much) not saturated or they will disintergrate. Sprinkle the Flubenvet lightly on the surface as you mix very thoroughly. Set the full 7 days worth aside and ONLY feed this during the seven days, no treats as Joy says. If you have any left over you can feed for a couple of days longer or discard any excess as you like but don't dilute it with unmedicated feed. Also don't mix corn or any other feed in with it as they may pick out the corn and not get the medication at the right concentration.
Worm more regularly if you keep the birds on the same ground all year round. Say 3 or four times and more frequently in the warmer periods say once in midwinter (dec), late spring (April), mid summer (Jul)and autumn (Sept). If you move your chx on a regular 3 month cycle you can reduce the worming to twice a year or just do it as the need arises by examining the dropping microscopically (nice job!) if you are sure you can spot the signs.
HF