The traditional method is to lay them flat on the ground and put holes in them to plant the tomatoes in - this doesn't give much depth of soil for the roots.
I suggest
1. Pummel/knead the growbag so it breaks up any lumps inside and spreads the soil evenly through the whole bag.
2. Lay the growbag on the floor as would normally, then pick it up in the middle (like an upturned letter "V") but with the sides closer together
3. Slit across the top to open up the compost (leaving the bottom bit joined so you have two halves or separating them completely is of course your choice)
4. This should give you 2 deep 'pots' to plant your tomatoes into.

Hope this makes sense, I can see what I mean in my own mind but putting it into words it sometimes loses something in translation.

Wouldn't it be just as easy to plant the tomatoes in large containers/pots filled with compost?

It would, but if you don't have any big pots............................

The method we use is to cut the bottom from plastic pots (8 inches across ones) and place them on the cut openings of grow bags. Fill the pots with compost and plant the tomato plants in the pots. A variation on the old ring culture system.

Good one OwdB

bought some open bottomed pots [as ring cuture]but this article i saw seemed to suggest some way of doing what rogerbee suggests but with the two halves on top of each other giving twice the depth ,thank you all

Rogerbee I followed that completetly so if I got it I am sure others did to. A very good explanation in my book. Well done.

i used large bukets from the d.i y shop £1 each and put holes in the bottem ,cheap er than garden cent . were they r bt 7 to ten quid each .or asder do them as well.