Unless you have really trashed the bearings its a 10 minute job as long as you have the little plastic Shimano widget for pulling the axle bearing assembly out.
Other than that all you will need is: solvent, rags, grease, 10 & 8mm spanners.
Pull the axle assembly out after you have unscrewed it with the widget, flush the bearings/collar unit out with solvent and the same with the pedal chamber.
Now see if the bearings have play in them, if they do its just the same as cup/cone wheel bearings to take the slack up. Once your happy the bearings are free of play without binding. 1/2 fill the base of the pedal chamber with grease & screw in the axle assembly the grease will force its way through the bearings & the collar unit squidging any excess out through the collar. refit & ride. It’s taken me longer to type this out than it takes me to service them.
Obviously if the bearings are rusty/really bollock’d then throw away and buy some new ones, bearing (see what I did there) in mind new cleats cost around £10 so if your getting the pedal & cleats for £18 then the pedals are in effect only £8
