Here are a few instructions on using tables in an old version of MS Word to set out your inserts. There are many other tools that can be used for this but I thought it may be the easiest. My screen prints are from an old version of MS Word, but the theory is the same whatever version you are using. It may seem complicated - but just work through the instructions, once you have set it all out you need never work it out again.

Working it out
First it is best to work out some basic maths and best to jot it onto a piece of paper,I always cut my inserts 1.5cm smaller than the total opened width, and 1 cm less then the total height.  For just printing on the card work with the full (opened) width of your card and the exact height. 

So for a basic card that is A5 folded in half eg 21cm X 15cm I would use a piece of paper 14cm high and 19.5cm across, this should give me approx ½cm all round as the insert will not go right to the middle.  This will fit across the top of an A4 piece of paper, anything wider than this will take up a whole piece of A4 paper set to landscape.

Continuing with the A5 sample as above I will plan to create a table with three columns (left hand side of the insert, centre margin and right hand side of the insert) the total page will consist, so this is my basic maths

Left margin (1cm)
The first column will be the middle of the left half (7.75cm)
second column will be the right margin of the left half AND the left margin of the right half (2cm)
third column will be the middle of the right half (7.75cm)
Right Margin (1cm)

This will make the total 19.5cm

So let's start setting things out in Word

Setting your margins
So click on File, --> Page Set up, -->Margins taband set your left and right margins to 1cm Make sure your top and bottom margins are the same value as each other 1cm is good at this stage - but unless you have a large insert it does not matter how much.

Still in the same pop up box click on -->Paper and choose paper size, select your size or custom (last selection) and then set the size of your paper - in this case width 19.5cm X height 14cm

Still in the same pop up box click on -->layout and set vertical alignment to 'center'. Now click OK .

Creating a table
Now you need to create a table - you may have a little box on your toolbar that is a checked box with a blue top - you can click on this and drag your mouse over three columns.



If not click on table, insert table, then choose 3 columns 1 row.



Now all you need to do is click on -->Table, then on -->Table properties click on -->Column, then put your column width to 7.75, 2 and 7.75 use the previous and next column buttons.

Now you are done. The only thing you may need to do is set your gridlines to not print, make sure your cursor is in the table, click on -->Format  -->borders and shading and click the top box on the left to 'none'.  You may have a short cut Icon if you have the Formatting toolbar on.

Make sure you save this template because you can use it every time.  Give it a quick test before spending time typing everything.

Add your message
Type your message in the third column. By hovering the mouse above the column it should change to an a arrow pointing downwards, click when it is like this and it will select the column. You can now make your text centred, and can select the font of your choice and the appropriate size and colour. Make sure that there are no blank lines above or below the text within the box, this will ensure it is centred.

If you can't see the table you have created try -->Table --> Show Gridlines.