Get a sheet of ply, Make a curcular template to the external measurement of the bricks if the sheets big enough, if not then do 2 half moon templates and then push them together and screw with a brace so basically there’s a hole in the middle. Then mix some mortar up, put a bed down on the existing and lay bricks ontop pushing them back to the template and follow it round. When you go up to the next level just put some bricks, timber or whatever underneath the template to hold it up and repeat. When you lay the next layer of bricks then make it so the joints don’t run in line so they’re staggered, if you can get some mesh, chicken wire, some sort of ties then can push this into the morta before laying the brick just to tie it in a little better but not necessary.
best advice I can give for a DIY. Won’t be perfect but may do the trick for you.
hope it helps.