@Fire I add it to the watering can (the recommended amount or slightly less) and water as I would normally (8l can used for 3 potted roses). I rarely do this for plants in the ground, they have DA slow release food.
EDIT (after I've seen your questions in your thread): I would say that the point of a liquid feed is to use it when you would water anyway, simply just add it to the can. So it's convenient and no added work. But if you use a hose or don't water at all, it wouldn't work for you.
I use a pressure sprayer and give foliar feed, works out more economical and easier for me to apply. If I had fewer roses I would do a root drench but just not practical for my situation. I’m trying Uncle Tom’s this year which seems to be having a good effect.
@Fire I lived in Vancouver in the winter and a small gulf island during the summer.
I'm also using one of those pressure sprayers for the foliar feed and. Once I use up my tomato feed I'm tempted to Uncle Tom's. But there's no getting around the fact I have 19 roses which need treating!
Teasing Georgia Mystery Rose, which we inherited, its only taken me 5 years to decide its a climber but it gets terrible blackspot and I keep threatening to dig it up.  The Generous Gardener with Madame Julia Correvan just opening. 
@Fire, I suffer with back and shoulder pain. I have 5 watering canes. I put them on bench so i dont have to bend to pick up. I put liquid feed in each. then fill them with water using hose. Then carry one at a time to water/feed plants (dahlia, potted rose, annuals and tomatoes etc).
Another method I used was to add liquid feed to water butt, then connect hose to it and use that hose to feed. It was very time consuming but easy on back and I used it for roses in ground couple of times.
I should switch to slow release food once I have used up all tomato feed stock. I have tried it once early spring this year.
My favourite part of the garden in spring (usually, it looks like this mid-May). If you want a small reliable allium, I can recommend this - Allium Unifolium Eros. The leaves stay green for a long time, die back well, 3-4 weeks flowering time, spreads/naturalizes, loved by bees. 'Jacqueline Humery', a pure white rose normally, opening in pink 'Eyes for You' and 'Purple Skyliner' sea of buds and lupins 'General Schablikine'
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EDIT (after I've seen your questions in your thread): I would say that the point of a liquid feed is to use it when you would water anyway, simply just add it to the can. So it's convenient and no added work. But if you use a hose or don't water at all, it wouldn't work for you.
I'm also using one of those pressure sprayers for the foliar feed and. Once I use up my tomato feed I'm tempted to Uncle Tom's. But there's no getting around the fact I have 19 roses which need treating!
Teasing Georgia
Mystery Rose, which we inherited, its only taken me 5 years to decide its a climber but it gets terrible blackspot and I keep threatening to dig it up.

The Generous Gardener with Madame Julia Correvan just opening. 
😖
Another method I used was to add liquid feed to water butt, then connect hose to it and use that hose to feed. It was very time consuming but easy on back and I used it for roses in ground couple of times.
I should switch to slow release food once I have used up all tomato feed stock. I have tried it once early spring this year.
If you want a small reliable allium, I can recommend this - Allium Unifolium Eros. The leaves stay green for a long time, die back well, 3-4 weeks flowering time, spreads/naturalizes, loved by bees.
'Jacqueline Humery', a pure white rose normally, opening in pink
'Eyes for You' and 'Purple Skyliner' sea of buds and lupins
'General Schablikine'
(I suppose that's sexist though, isn't it, as I'm assuming that the General is a male)