Be aware that by adding just additional extract, you also increase the FG, which leaves the beer sweeter. Adding just sugar, you lower the FG (making it dryer). You need to strike a balance, and a combination of both is probably the right thing to do.
What proportions? I don't know that there is any guidance. I would say add something like 50/50 extract/sugar. This assumes you are not adding too much; maybe 0.5 lbs DME and 0.5 lbs table sugar. This would add .009 to the OG of 5 gallons or an a little over 1% abv.
Limit sugar to about 5% of the total fermentables unless the beer style permits more (some English, and Belgians mainly). 5% in 5 gallons of 1.060 wort would be about 10 ozs of table sugar, or 13 ozs of corn sugar.
An earlier poster said to use corn sugar. I see no reason not to use plain table sugar. I use it all the time. It's what is used in English beers, used in some Belgian beers, and the basis for candi sugar. Corn sugar is an American invention, does not make any difference to the beer, as far as I know it is not used in any commercial beers, and is over-priced compared to other sugars.