It would be ideal to come and measure in you home bit this is not always possible due to location so there are a few simple guidelines to help you with measuring. Fitting of poles, curtains and blinds are not covered in the cost however we can recommend experts.

Curtains
Roman blinds in a recessed window (blind fitted in the recess)
Roman blinds in a recessed window (blind fitted outside the recess)
Roman blinds in a non recessed window (blind fitted to the architrave)
Roman blinds in a non recessed window (blind fitted to the wall above the window)
Roman blinds in a bay window.
How do I know which type of window I have?

Measuring for curtains

Start by defining the width of the curtains. To do this measure the curtain track or pole using a steel rule. If you have a curtain pole please make sure that you measure between the finials (decorative bits at the end of the pole)
Decide whether you want a pair of curtains or a single curtain
Decide what kind of curtain heading you want to use - pencil pleat, tab top etc.
Determine the finished drop of the curtain, whether to the sill, to below the sill or to the floor
Pencil Pleat - measure from the top of your curtain track or the bottom of your pole ring to where you want the curtains to end. This is the finished drop and we will make the calculations from this. Remember that if you want the curtains to the floor then measure to 1cm above the floor for your finished drop

Measuring for Roman blinds

What type of window do you have?

Does it recede deeply into the wall to a depth of at least 10 cm, with the windowframe laying flush to the wall outside? Probably with a deep window sill. If it does you have a recessed window.
Does the frame sit in line with the wall on the inside ie in the room. Probably the windowsill is very shallow. If it does, you have a non-recessed window.
If you want a shaped or zigzag bottom, note that the drop measurement includes the bottom section.

Measuring in a Recess

A window in a recess is where the window is 'sunk in' to the wall. You have two options:

Option (1) Fit the blind in the recess

Advantages:

It makes for a very neat finish.
The blind batten can be fixed onto the window frame if it is wood, which is an easier fixing to do.

Disadvantages:

When pulled up, the blind will obscure part of the window allowing less light through, which may be a problem if the window is small.
If the bottom of the blind is shaped, light will come through at the base of the window when the blind is down. This is only really a consideration if the window is in a bedroom.

To measure the recess:

Take the measurement from the top to the bottom of the recess. Do this at either end of the window and once in the middle if the window is wide, to check for irregularities. Take the smallest measurement and deduct 1 cm from this. This is the length, or 'drop' of the blind.
Next, measure from one side of the recess to the other side. Again, do this in more than one place to check for irregularities and take the smallest measurement. Deduct 1cm from this- this is the width of your blind.
Be careful when measuring the width, of anything that will affect this measurement eg picture or dado rails that come into the recess or thick tiles in kitchens or bathrooms that come part-way up the recess. Obviously, such things will make the width of the blind that much narrower.

Option (2) Fit the blind outside the recess onto the surrounding wall.

The advantages are:

It gets around the problems of picture or dado rails as discussed above, or inward opening windows. It also won't affect ornaments etc if these are placed on the sill.
As the blind batten is fixed above the recess, the blind will fold up fully or partly above the window and will not affect light levels so much.
As the blind bottom comes down below the window, if it is shaped, it will still keep the light out ( a potential issue in bedrooms).

The disadvantages are:

The blind batten may be going into a concrete lintel which is a harder fixing.
If you have objects you wish to display in the recess, then they will be obscured from view when the blind is let down.

To measure outside the recess:

Decide how far outside the recess the blind will extend - 5 to 10cm either side. This is the width of your blind.
Work out where your blind bottom will sit - this will usually be 10-20cm above the window. Generally, you will want the blind when folded, so sit above the window, but to cover the top part of the windowframe. Measure from this point to 5-10cm below the recess. This is the length of the blind.

Measuring a non-recessed window

A non-recessed window is one that lies flat against the wall. You have two options:

Option (1) Fix To The Architrave

Blinds on these windows can usually be fixed onto the wood architrave surrounding the window. Fixing the blind batten into wood is always easier than fixing into the wall. You can choose to have the blind drop to the windowsill or below it (if the sill does not jut out too much). You might want to drop below the windowsill for aesthetic or light reasons.
For the length, measure from the top of the architrave to the top of the sill or appoint 5-10cm below it (A). Measure twice on either side for irregularities and take the smallest measurement if there is a discrepancy.
For the width, measure from the outside edge of the architrave across to the other side (B). Measure twice for irregularities, but this time if there is a difference take the larger measurement.

Option (2) Fix To The Wall Above The Window

If there is no wood around the window, you will need to fix to the wall above. You may wish to make your blind bigger than the actual window size.
To measure the length, work out where your blind batten will sit - either just above the frame so that he blind will fold up and cover the top part of the window, or 10-20cm above the window so that when folded only the bottom folds of the blind cover the top of the windowframe. Measure from this point to the windowsill or 5-10cm below it (A) (depending on whether you want to obscure the windowsill or not). Measure twice on either side for irregularities and take the smallest measurement.
For the width, decide how far outside the frame your blind will extend - 5cm to 10cm according to choice - and take this measurement (B). You do not have to overlap 5cm to 10cm in this way - you could just make the width of the blind the same as the windowframe depending on personal taste.

Bay Windows

Bay windows are never considered to be in a recess - you have to look them as non-recessed and look to fitting in one of the two ways described above. The issues that arise are:

Batten length: Extra care needs to be taken that you don't order blinds that are too wide, particularly for the central section.
Gap: You want a cm or two gap between