To weigh her, your vet surgery should have scales you can use. They may even have a free/cheap weight clinic to advise you.
While my dogs have never been very over weight, there have been times Murphy has carried a little extra weight and currently Buffy is a shade heavier than she really should be. I've gotten away with just upping the exercise and using lower calorie treats.
You need to know the calorie intake your dog needs - this would be less for a spayed bitch than entire. Then feed her slightly less. Any treats count towards the daily calorie total. Weigh everything out in the morning then spread the food over the number of meals & training sessions she needs.
As mentioned, a lower calorie food may give her a bit more food for the number of calories she needs.
As important, if not more, is the exercise. Can she go off lead? (I know some beagles can't because their recall is extremely selective) If not, invest in a 'long line' or extender lead. This way she'll get all that running around dogs love without the risk of loosing her. If you have a partner/relative/friend who can walk with you, you could play scenting games with her. Hounds love to follow a scent. But an adult beagle is going to need a good couple of hours exercise a day anyway.
Just seen your latest post - Don't know why vets recommend Chappie

As main stream foods go, it actually isn't the worst (apart from the flatulence!!!!!) But there are
much better foods out there. Burns, Orijen, CSJ, even Wainrights. Actually Wainrights (Pets@home own brand) light may be a good choice - low cal and not too expensive.
And garden exercise is nothing compared to walking. Get an extender/long line and get her to some country parks/fields/woods. Unless you walk her seriously, she will never loose weight.
Bottom line, reduce her calorie intake and increase her exercise.