Replacement windows, as now understood, usually means that the entire window frame is replaced by a new frame with a sealed double glazed unit. There is not really any DIY work involved, unless you want to screw the new frame into place. Where, however, you may want to do itt yourself is in a situation where you wish to keep the original window frames, usually because they are in keeping with the house's age and history. In this scenario, you can put double-glazed glass into the frames yourself, provided they are hardwood frames and have sufficient depth to allow you to widen the rebate. The double glazing units themselves need to be made by a specialist firm, who will make them to your specificatDo It Yourself Replacement Windowsions, but all the joinery and fitting work you can do yourself. The glass units will come with glazing compound and spacers (the little bits of rubber). The glazing compound is a modern equivalent of putty. It is considered preferable to putty because it should not grow hard and dry. Many builders, however, still prefer working with a linseed oil based putty. The choice is yours!

If it is a small frame and you can remove it and do the work in a workshop, this will be easier, but this is not advisable with large windows, where the weight of the double-glazed window would make it unmanageable.

To remove the old glass, first take off the external beading that surrounds the glass. It will almost certainly be impossible to do this without breaking the beading. Don't worry about this. It is better in any case to put in new, fresh beading with the new windows. Once you have removed the beading, chip away the old putty and, with a fine pair of pliers, remove all the tacks that are holding the window in place. It will not move at this stage and you can quite confidently take out each tack.

The next stage needs two people (at least). Make sure you are wearing strong protective gloves as, when the glass starts to move, you need to be able to handle its edges. The person outside should hold the glass firm with two suckers on wooden handles; the kind used for clearing blocked drains are fine. Wet them before applying them to the glass – this gives a stronger vacuum hold. The person inside should then start to tap the glass gently with a rubber mallet, starting at the top. The glass will start to move and you will eventually be able to lift it right out.

Now enlarge the rebate to allow for the width of your units. Initially, draw a pencil line along the frame, drill holes right along this line, the depth of the rebate, and then drill a corresponding line of holes on the horizontal plane, again the depth of the rebate. Take out the waste wood with a chisel. Be careful not to chop out too much wood at a time in case the chisel is driven over the line of the rebate. Once the waste wood is mostly out, use your chisel first to take out inequalities and then use a shoulder plane to trim the rebate smooth. Finish smoothing the rebate with sandpaper – a rectangular wooden block with the sandpaper wrapped round it is the obvious way to do this.

Apply two coats of silicone sealant to the rebate and allow them to dry. Next apply a thick coat of the glazing compound. Put spacers, 1ft apart, along the bottom of the rebate. Place other spacers, 1ft apart, against the back of the rebate. This is to prevent the glass moving. Set the sealed unit into the frame, making sure that you are pressing it firmly into place. Once the glass is in place, lay an outer layer of the glazing compound, with spacers put on the bottom of the rebate and against the glass at 1ft intervals, just as you did with the inner layer of compound.

The final stage is to press the beading hard into the glazing compound and up against the spacers. Screw the beading into place. Make sure the screw heads are countersunk. Clean excess glazing compound off the glass. Cover each countersunk screw head with a little compound and when it is dry, sand it smooth.