This is the wonder and beauty of the information age. You have a an abundance of people all clamouring for your attention online. Youtube is full of a world of instruction. Today you can watch and listen how others take on just about any DIY project on YouTube.
Sometimes it is better to involve a professional; someone with deep knowledge and skill can offer untold benefit to any project you wish to encounter. This can be done by offering to hire out the project and by getting contractors to give you a quote, and some idea of all that needs to be done to complete any given task. For example by itemizing what work they will do for that sum of money. Getting prices from various qualified trades people will help you determine the level of skill necessary to complete the task and give you a costs vs. benefits view of the project at hand.
But for those brave souls who wish to DIY we recommend thoroughly investigating the situation, watching youtube videos and at least speaking to as many knowledgable people that you can about your chosen project.
Depending on your level of skill and knowledge sometimes it is still better to turn the project over to a skilled trades person in order to save time and even money, given the cost of flooring products today.
Making sure that your substrate is sound, (doesn't move when you walk on it) is very important. Also make sure that the floor you are covering is smooth. The smoother you can make the subfloor the more integrity the finished floor, your work, will have.
Re-secure you subfloor whenever possible.
If the upper sheet old plywood has been removed as well as the old floor covering, you have an excellent opportunity to tighten up the lower floor boards on the floor joists, making sure that everything is solid and secure.
Re-securing the floor boards to the joists, while you can, will go long way to keep your floor free of squeaks for years to come. If nails are used they should be 1 3/4 ring nails, or even better use a 1 1/2" flooring or wood screw with a lithium-ion drill, to tighten up your floor boards on the joists.
I re-secured each floor board (5/8' tongue and groove) into each joist 25 years ago. Not only are there no squeaks but the floor is much more sound.
Once your base ply-wood is secure you can install the 4' x 8' x 3/8" underlayment. Fir Good 1 Side or 3/8 Fir G1S is recommended. There are other products out there that are less expensive but the 3/8" Fir is just a little thicker and offers just a little more dimension stability, than either Luan Mahogany or Poplar. I've used them all and given the project they are inter changeable but the Fir G1S is the best, IMO
I've used both ring nails 1 1/4, underlayment screws with a screw gun and staples with a special 11/8 staple gun and a compressor.
The important points in fastening underlayment to the substrate properly are:
1.) Make sure the head of your fasteners are slightly buried beneath the surface of the wood, so the don't photograph through your finished floor covering.
2.) Fasten at 2 inch intervals on the perimeter of each sheet and then on 4" inch centres in both directions within the interior of each sheet. This is important to make sure your underlayment is fastened to the substrate in this manner. If you miss areas with the stapler you will have the underlayment expanding and bubbling up in areas.
Use a cement based floor patching compound mixed with a latex additive to feather the joints of the underlay so as to make the seams invisible through the finished floor covering.